[go] tests/GoTest.sh: Fix flags.Parse location to work on new go SDKs. (#6388)

[go] tests/GoTest.sh: Fix flags.Parse location to work on new go SDKs. (#6388)

* tests/GoTest.sh: Fix flags.Parse location to work on new go SDKs.

Calling flags.Parse() within init() races with other packages which register
flags in their init(), and in particular with the testing package itself. It is
more reliable to call flags.Parse() from a TestMain implementation.

See https://github.com/golang/go/issues/31859,
https://github.com/golang/go/issues/33869.

* .github: Enable build-go action in build.yaml workflow.