workflows: move eval from push to queue

Running eval in the merge queue prevents eval failures after merging
multiple PRs with semantic merge conflicts.

It's also the base for allowing more sophisticated checks about rebuild
counts in the merge queue later.

When branches are directly pushed to, such as for periodic merges, this
will not run Eval immediately. However, the next PR will run it as part
of its Eval step and will cache the results. Any further PR will then
benefit from the same caching again.

This also avoids running Eval twice when pushing to staging-next or
haskell-updates, where a PR is open at the same time. Here, the PR's
Eval still runs on the push, of course.

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