Fix is_maintenance_branch heuristic

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: https://git.lix.systems/lix-project/lix/issues/443
Change-Id: I537103ebab58ae978c00e06972abe14432dd9c80

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