x/build/cmd/coordinator: don't try trybots for oauth2, even for 1.10 and 1.11 branches

[bradfitz]

See https://go-review.googlesource.com/c/oauth2/+/157137 in which we run exactly 2 trybots for oauth2: the 1.10 and 1.11 branches, but not tip or anything else.

This is a regression from fixing #17626

That oauth2 (or packages with non-golang.org/x/ deps) isn't compatible with the coordinator is #14594

/cc @dmitshur @bcmills

[gopherbot]

Change https://golang.org/cl/157438 mentions this issue: cmd/coordinator: support testing subrepos that use modules