Backporting

To automatically backport a pull request to a release branch once it's merged, assign it a label of the form backport <branch>.

Since GitHub Actions workflows will not trigger other workflows, checks on the automatic backport need to be triggered by another actor. This is achieved by closing and reopening the backport pull request.

This specifically affects the installer_test check. Note that it only runs after the other tests, so it may take a while to appear.