x/build/cmd/coordinator: newTrySet provides invalid goBranch value to dashboard.TryBuildersForProject when testing custom branches of golang.org/x repos

[dmitshur]

cmd/coordinator makes an assumption that is not safe to make here:

https://github.com/golang/build/blob/39561bf34556f888d43bce0ed218426494ecd88c/cmd/coordinator/coordinator.go#L1167-L1169

This is because dashboard.TryBuildersForProject may not return any builders when given a non-existing goBranch value, or it may return the wrong ones.

Spotted while reviewing CL 227397, but it's not critical to fix for #37512 because dashboard.TryBuildersForProject doesn't filter out by goBranch too aggressively at this time. Filing an issue so we don't forget.

/cc @cagedmantis @toothrot