Make our release Kokoro build more flexible
Make our release Kokoro build more flexible

This will allow us to perform just *bits* of a release, using a commit other than the one that the Kokoro script itself was fetched at.

In particular, it should allow us to fix #3837 by performing the releases again, specifying the tags as environment variables (and skipping NuGet push and GitHub pages push).

jskeet/gcloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet