* Pin CI runner images to explicit versions
CI is run infrequently enough on this repository that there's a
nontrivial chance that GitHub updates the definition of `ubuntu-latest`
for example between two CI runs. This can be confusing for contributors
as PRs seemingly break CI when in reality they have nothing to do with
the breakage and it's due to the image changing.
This PR pins all images to the definition of `*-latest` at this time.
The one exception is that one builder was pinned to 22.04 and I'm going
to update it to 24.04 here and see if I can't fix CI issues that come up.
This'll require explicit PRs to update these images in the future, but
hopefully that's only once every few ~years so not too much of a burden.
* Try not custom-installing clang 18
* Try to fix CI issue
* Fix CI again (attempt)
* Further try to fix CI
* Another fix for CI
Surely if I keep making commits that say "another fix" eventually I'll
run out of fixes. Surely, right? I'd put this in limerick form to be
more amusing if someone comes to read this but I'm not clever enough to
do that, so instead I'll just abruptly
Alex Crichton
e6a833783a