cmd/releasebot: run tests in mode=release for betas
cmd/releasebot: run tests in mode=release for betas

Beta releases release from master. We cannot be certain that a change
hasn't been merged between prepare and release.

Fixes golang/go#36171

Change-Id: Ic8d043fa11259a7f8b307f6387e7b2b702ce1046
Reviewed-on: https://go-review.googlesource.com/c/build/+/211583
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build