x/build/cmd/coordinator, x/build/maintner/maintnerd/maintapi: golang.org/x repo trybot on custom release branch should use corresponding Go branch

[dmitshur]

When trybots are testing a CL in a golang.org/x to a release branch with a custom suffix (e.g., release-branch.go1.14-cmd or release-branch.go1.15-bundle), they should use the mentioned Go version to test that CL, not tip.

See CL 264058 where Go tip (1.16) was used, despite the release branch having a "go1.15" in it.

This is somewhat related to issues #37512 and #38303. Custom release branches are somewhat unusual (now for #41375, last time for #36851) and hopefully won't come up much in the future.

CC @golang/release.

[dmitshur]

The problem is on this line:

https://github.com/golang/build/blob/247680342f28bf27b68358c64c491da551864749/cmd/coordinator/coordinator.go#L1220

When a release branch has a suffix (like "-bundle"), the branch == work.Branch equality won't hold.

A fix to allow suffixes separated by a dash could be to change that to work.Branch == branch || strings.HasPrefix(work.Branch, branch+"-").

[gopherbot]

Change https://golang.org/cl/264203 mentions this issue: cmd/coordinator: use right Go for x repo release-branch with suffix

[gopherbot]

Change https://golang.org/cl/319789 mentions this issue: cmd/coordinator, maintner/maintnerd/maintapi: consolidate TryBot branch logic