Do not export GOROOT

This has not been necessary since [Go 1.9](https://go.dev/doc/go1.9#goroot) at least (although clunky) but definitely not since [Go 1.10](https://go.dev/doc/go1.10#goroot).
This is cargo culting code that is more than 2 years out of date and runs into issues when multiple go versions are used in an action run.

Signed-off-by: Manuel Mendez <mmendez534@gmail.com>

__tests__/setup-go.test.ts

@@ -243,7 +243,7 @@ describe('setup-go', () => {
     });
 
     await main.run();
-    expect(vars).toBe({'GOROOT': 'foo'});
+    expect(vars).toBe({});
   });
 
   it('finds a version of go already in the cache', async () => {

src/main.ts

@@ -26,7 +26,6 @@ export async function run() {
 
       const installDir = await installer.getGo(versionSpec, stable, auth);
 
-      core.exportVariable('GOROOT', installDir);
       core.addPath(path.join(installDir, 'bin'));
       core.info('Added go to the path');