Flag --incompatible_use_platforms_repo_for_constraints will break rules_go in Bazel 1.2.1

[bazel-flag-bot]

Incompatible flag --incompatible_use_platforms_repo_for_constraints will break rules_go once Bazel 1.2.1 is released.

Please see the following CI builds for more information:

• [:windows: (OpenJDK 8)](:windows: (OpenJDK 8))
• [:ubuntu: 16.04 (OpenJDK 8)](:ubuntu: 16.04 (OpenJDK 8))
• [RBE (:ubuntu: 16.04, OpenJDK 8)](RBE (:ubuntu: 16.04, OpenJDK 8))
• [:darwin: (OpenJDK 8)](:darwin: (OpenJDK 8))
• [:ubuntu: 18.04 (OpenJDK 11)](:ubuntu: 18.04 (OpenJDK 11))

Questions? Please file an issue in https://github.com/bazelbuild/continuous-integration

Important: Please do NOT modify the issue title since that might break our tools.

[jayconrod]

I think all the rules_go work is done for this flag, but we're blocked on bazel_skylib. It was migrated in bazelbuild/bazel-skylib#214, but that commit is not released yet.

@aiuto Is there a bazel_skylib release planned soon that will include that fix?