Cirrus: Only upload tagged releases

Prior to this commit, every push to master had it's builds packaged and
uploaded to google storage.  This is a waste, since potential users
are only ever concerned about tagged releases.

Unfortunately because the release process involves humans with
potentially multiple human and automation steps happening in parallel,
it's easy for automation to not detect a tagged release, or trigger on
development|pre-release tags.

Fix this in `upload_release_archive.sh` using a new unit-tested
function `is_release()`.  This acts as the definitive authority
on whether or not a specific commit rage or `$CIRRUS_TAG` value
constitutes something worthy of upload.

Signed-off-by: Chris Evich <cevich@redhat.com>

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