With the latest performance improvements, this only adds up to 1s, but
reduces complexity in managing pinned checkouts explicitly, especially
when we need *both* pinned checkouts in the next commit.

Instead of uploading the outpaths as artifact, this uploads them via
cachix. Most of all, this makes CI less brittle, because Eval in PRs
will still be able to succeed, even if no workflow run for the push
event could be found on the target branch. It will just take longer.

This also makes moving Eval into the Merge Queue easier to do: When
downloading artifacts from a different run, these would always have to
match on the right event, too. By pulling from cachix, the same workflow
can support target branches with merge queue and without merge queue at
the same time. The latter would still use the push event, while the
former could use the merge_group event.

Last but not least, this should fix Eval on PRs targeting `wip-`
branches and any other branches that the push event doesn't trigger on.
These would never find an Eval result from the target branch and could
never show rebuilds accurately. Now these PRs should work at a slightly
higher runtime cost.