Update to use hasNextSupport for custom-routes in next export check

[ijjk]

Follow-up to #17538 per #17538 (comment) this updates the check to use the existing hasNextSupport export instead of checking the environment variable directly

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
buildDuration	13.6s	13.6s	⚠️ +37ms
nodeModulesSize	63.2 MB	63.2 MB	⚠️ +67 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.549	2.599	⚠️ +0.05
/ avg req/sec	980.83	962.05	⚠️ -18.78
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.526	1.624	⚠️ +0.1
/error-in-render avg req/sec	1638.21	1539.42	⚠️ -98.79

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
677f882d2ed8..9339.js gzip	10.9 kB	10.9 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-d493e7d..42f5.js gzip	7.17 kB	7.17 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	57.8 kB	57.8 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
677f882d2ed8..dule.js gzip	6.77 kB	6.77 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-f8905d4..dule.js gzip	6.24 kB	6.24 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	52.7 kB	52.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-d2344ce..8b36.js gzip	1.3 kB	1.3 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.71 kB	7.71 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-f8c0daf..dule.js gzip	1.26 kB	1.26 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.36 kB	5.36 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
_buildManifest.js gzip	322 B	322 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	651 B	651 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
index.html gzip	1 kB	1 kB	✓
link.html gzip	1.01 kB	1.01 kB	✓
withRouter.html gzip	996 B	996 B	✓
Overall change	3.01 kB	3.01 kB	✓

---

Serverless Mode

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
buildDuration	15.2s	15.1s	-103ms
nodeModulesSize	63.2 MB	63.2 MB	⚠️ +67 B

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
677f882d2ed8..9339.js gzip	10.9 kB	10.9 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-d493e7d..42f5.js gzip	7.17 kB	7.17 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	57.8 kB	57.8 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
677f882d2ed8..dule.js gzip	6.77 kB	6.77 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-f8905d4..dule.js gzip	6.24 kB	6.24 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	52.7 kB	52.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-d2344ce..8b36.js gzip	1.3 kB	1.3 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.71 kB	7.71 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-f8c0daf..dule.js gzip	1.26 kB	1.26 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.36 kB	5.36 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
_buildManifest.js gzip	322 B	322 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	651 B	651 B	✓

Serverless bundles

	vercel/next.js canary	ijjk/next.js update/export-custom-routes-check	Change
_error.js	1.05 MB	1.05 MB	✓
404.html	4.34 kB	4.34 kB	✓
hooks.html	3.92 kB	3.92 kB	✓
index.js	1.05 MB	1.05 MB	✓
link.js	1.1 MB	1.1 MB	✓
routerDirect.js	1.09 MB	1.09 MB	✓
withRouter.js	1.09 MB	1.09 MB	✓
Overall change	5.4 MB	5.4 MB	✓

Commit: 5c585e7