cmd/genbuilderkey: migrate secrets to secret manager
cmd/genbuilderkey: migrate secrets to secret manager

This change retrieves the master builder key from secret manager. It
is part of the project to store all secrets in a single location.

Updates golang/go#37171

Change-Id: I0c8b8fe8a3db5b9583008bfc105391eca69fba78
Reviewed-on: https://go-review.googlesource.com/c/build/+/222958
Run-TryBot: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

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