gh-36338: CI: Merge open blocker PRs in all CI workflows + other improvements
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The new script `.ci/merge-fixes.sh` uses the GitHub CLI to retrieve open
PRs with the "blocker" label from sagemath/sage and tries to merge them
into the tested branch, ignoring any PRs that give merge failures.
We call this script in all CI workflows. Thus,
- hotfixes for the linter (such as #36327) or
- fixes for build problems (such as #36276)
will take effect in the CI workflows of all PRs.
This can be seen in the runs of the workflows of the present PR (section
"Merge CI fixes from sagemath/sage").
Additionally,
- we fix `debian-trixie`, which failed because a system package is no
longer available
- we fix `macos-conda-maximal` by removing some fictitious
distros/conda.txt files
- we refactor ci-macos using a new reusable workflow; example runs:
- https://github.com/mkoeppe/sage/actions/runs/6308124398
-
https://github.com/sagemath/cysignals/actions/runs/6316714387?pr=185
- we update the ci-macos platforms, including a configuration for the
new and problematic Xcode 15.0
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/36338
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik, Jonathan Kliem, Kwankyu Lee, Matthias Köppe, Tobias Diez
