actions/checkout: parallelize checkout of multiple commits on tmpfs

Instead of fetching up to 3 times on each new checkout, we now fetch all
the commits we're going to need at once. Afterwards, we checkout the
different worktrees in parallel, which doesn't give us much, yet,
because it would still be IO-bound on its own. Inconsistent IO
performance on disk is also the biggest limitation for checkout right
now, where checkout times range everywhere from 20s to 40s.

By checking out the worktrees on a tmpfs, the actual checkout only takes
1s and benefits from parallelization. The overall checkout time is now
8-11s, depending on the number of commits.

That's a reduction of 10-30s and we get this speedup for almost every
job in the PR workflow, which is huge.

This potentially has a nice side-effect for Eval, too: Because the repo
is in RAM, Eval seems to run slightly faster, up to 10 seconds less.

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