MAINT: CircleCI build merged PRs #20658

Describe the issue

CircleCI (unlike Travis, AppVeyor, Azure, GitHub Actions, etc.) by default just builds PR branches themselves, as opposed to those PRs merged with upstream/master. This forces people to rebase to deal with issues that crop up, for example in #20585 (comment), and also decreases uniformity across CIs.

Proposed fix

For a while (years?) in MNE now we have manually merged pull requests after the checkout step. I'm happy to port our code if people agree it would be useful. The only time it breaks or dies is when there are merge conflicts (i.e., the code can't be successfully merged), which arguably is fine behavior because people should merge/rebase at that point anyway.

EDIT: Also need these lines