Only test release builds on main (#14475)
This is one of the slowest remaining jobs in the pull request CI. We
could use a larger runner for a trivial speed-up (in exchange for $$),
but I don't think this is going to break often enough to merit testing
on every pull request commit? It's not a required job, so I don't feel
strongly about it, but it feels like a bit of a waste of compute.
Originally added in https://github.com/astral-sh/ruff/pull/11182
