Switch GitHub Actions to checking out the PR tip by default

[sunshowers]

On running a pull_request action, GitHub Actions, in a decision which appears to be somewhat surprising at first, does not check out the pull request tip. Instead, it checks out a merge commit of the PR tip with the target branch as of the time the PR was created. See actions/checkout#426 for more discussion.

Buildomat currently always uses the PR tip.

While there are reasonable arguments and trade-offs in both directions, the most important thing is to make sure that Buildomat and GitHub Actions use the same scheme. The most straightforward way to do that is to switch GHA to using the PR tip.