Fix the backwards-compatible go_repositories() (#765)
Fix the backwards-compatible go_repositories() (#765)

Currently if you call go_repositories(), it fails with "no matching
toolchains found", as calling go_register_toolchains(go_version=None)
overrides the default value.

bazelbuild/rules_go,bazelbuild/rules_go,Xjs/rules_go,bazelbuild/rules_go,Xjs/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,Xjs/rules_go,Xjs/rules_go,Xjs/rules_go