x/build/cmd/releasebot: Beta release should always run tests

[toothrot]

The releasebot skips running tests in mode=release, as we have normally run tests on the commit we're about to release. For beta releases, it's possible that a commit has been merged to master between the time we've run mode=prepare and mode=release.

[gopherbot]

Change https://golang.org/cl/211583 mentions this issue: cmd/releasebot: run tests in mode=release for betas

[toothrot]

Long-term solution captured in #36182

[gopherbot]

Change https://golang.org/cl/235777 mentions this issue: cmd/releasebot: use same master commit for beta releases