Ensure appDir is included in webpack configVars (#42710)

This ensures the webpack cache is updated when toggling between `appDir:
true/false`.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/build/webpack-config.ts

@@ -2234,6 +2234,7 @@ export default async function getBaseWebpackConfig(
   }
 
   const configVars = JSON.stringify({
+    appDir: config.experimental.appDir,
     crossOrigin: config.crossOrigin,
     pageExtensions: pageExtensions,
     trailingSlash: config.trailingSlash,