Automatic Backporting Process with GitHub Actions

1st chunk of `maintainers/backporting.md`

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# Backporting

To [automatically backport a pull request](https://github.com/NixOS/nix/blob/master/.github/workflows/backport.yml) to a release branch once it's merged, assign it a label of the form [`backport <branch>`](https://github.com/NixOS/nix/labels?q=backport).

Since [GitHub Actions workflows will not trigger other workflows](https://docs.github.com/en/actions/using-workflows/triggering-a-workflow#triggering-a-workflow-from-a-workflow), 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.

Title: Automatic Backporting Process with GitHub Actions

Summary

To automatically backport a merged pull request to a release branch, assign it a label in the format `backport <branch>`. Due to GitHub Actions limitations where workflows don't trigger other workflows, checks on the automatic backport, particularly the `installer_test`, need to be manually triggered by closing and then reopening the backport pull request. The `installer_test` runs after other tests and may take some time to appear.