When we build a release, it is always a "past" commit - typically, one
that has already been tested twice: once when the corresponding PR was
run, and then again as a "main"-branch commit.
Release branches don't run, but their protection rules enforce linear
merges.
Either way, we know we're building a _good_ commit, and, assuming our
builds and tests are hermetic, testing that commit again when we make a
release is a pure waste of time and CPU resources.
The other case, where we make an ad-hoc release from a branch that has
not been merged, has a similar issue: we do not necessarily want to run
the full test suite, because part of the reason we need that commit may
be that it doesn't succeed as is.
Based on that observation, I wondered what might be the minimal set of
things we actually need to build when making a release. This PR is an
experiment in trying to find that out.