This was broken because Nix language's version comparison does not know
how to deal with versions like -rc1 and considers them newer, which is
in this case not desirable.
That in turn led to not tagging 2.90.0 docker images as "latest" since
the heuristic was wrong.
This commit also adds some more cross-checking and failsafes in case the
person running releng does not have a local main branch that is up to
date.
Fixes: #443
Change-Id: I537103ebab58ae978c00e06972abe14432dd9c80
For example, when releasing from release-2.90, if `main` has a 2.91 tag
ancestor, we know that 2.91 was released, so we should *not* tag latest.
Change-Id: Ia56b17a2ee03bbec74b7c271c742858c690d450d
jade