Improve PR instance-sync mechanism

[grst]

          > wouldn't the solution be to force-push to this (i.e.pr-390 branch) instead of changing anything related to the main branch?

yeah: force-reset the instance repo to current main, rsync template rendering over it, git commit, force push.

Originally posted by @flying-sheep in scverse/cookiecutter-scverse-instance#195 (comment)