ci: use custom PR message action

[agoose77]

I tried to use the RTD action in #1812, but it transpires that this leverages the RTD GitHub integration that we aren't using. We couldn't see this until it made it into a base branch, so I only discovered it after the merge.

So, this PR

• Roll our own PR message editing machinery.
• Replace Python with bash (avoid Python step in job).
• Bundle new JS to edit commit message.

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/agoose77-docs-disable-pr-link/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1816 (6beb189) into main (2511d4b) will decrease coverage by 0.00%.
The diff coverage is n/a.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/indexedarray.py	79.76% <0.00%> (-0.19%)	⬇️
src/awkward/contents/recordarray.py	83.72% <0.00%> (-0.18%)	⬇️
src/awkward/_util.py	82.36% <0.00%> (-0.18%)	⬇️
src/awkward/contents/indexedoptionarray.py	88.92% <0.00%> (-0.16%)	⬇️
src/awkward/_connect/avro.py	87.17% <0.00%> (-0.14%)	⬇️
src/awkward/types/_awkward_datashape_parser.py	47.72% <0.00%> (-0.01%)	⬇️
src/awkward/contents/content.py	77.31% <0.00%> (+0.10%)	⬆️

[jpivarski]

I would have asked if we should switch to the GitHub ReadTheDocs Integration, but you've already adapted to the interface we have.

I'll merge this and we'll see if it works. I got your message that you intentionally cancelled the build because it isn't testing anything; I'll use the red button to merge without successful tests.

(Fingers in ears...)