x/build,cmd/link: darwin/amd64 builders unexpectedly skipping `cmd/link.TestBuildForTvOS`

[bcmills]

In #58221 we observed consistent test failures in cmd/link.TestBuildForTvOS on darwin-amd64-11_0, but not on the other darwin/amd64 builders. The root cause of the failure is not OS-dependent, which suggests that the test is being skipped on all of the other darwin/amd64 builders. That is unexpected.

We should probably:

• adjust those builders so that they can run the test (attn @golang/release), and
• adjust the test so that it fails instead of skipping if GO_BUILDER_NAME is non-empty (attn @golang/compiler, @golang/ios).

(The discussion on #58221 covers more detail about the skips.)

[cherrymui]

Yeah, the test requires tvOS SDK installed on the machine. Not sure they are...

[heschi]

cc @prattmic for the next time we build macOS images?