ci/github-script/prepare: identify real base branch

When a contributor mistakenly sets the wrong target branch for a Pull
Request, this can lead to bad consequences for CI. Most prominent is the
mass ping of codeowners, that is already handled in
`ci/request-reviews/verify-base-branch.sh`. But there are other things
that go wrong:
- After eval, a mass ping of maintainers would still be possible, in
theory. Practically, this doesn't happen, because we have a limit of 10
reviewer requests at the same time.
- This will most often contain a change to `ci/pinned.json`, thus the
full Eval matrix of all Lix/Nix versions will be run, burning a lot of
resources.
- The PR will be labelled with almost all labels that are available.

We can improve on the current situation with some API calls to determine
the "best" merge-base for the current PR. We then consider this as the
"real base". If the current target is not the real base, we fail the
prepare step, which is early enough to prevent all other CI from
running.

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