Reuse built clang-tidy plugin across the shards

[moxian]

Summary

Infrastructure "Reuse built clang-tidy plugin across github action shards"

Purpose of change

This is a followup to #78801 (#78801 (comment))
It is a bit silly to re-build and re-test our clang tidy plugin thrice for each workflow run when we can do it only once instead. This PR does so.

Describe the solution

At a high level: split our matrixed clang-tidy job in two

• the first one (non-matrixed) builds the plugin, tests it, and uploads the artifact
• the second one (matrixed) downloads the plugin and runs it on the codebase (once per matrix shard, so thrice total)

This resulted in a split of ./build-scripts/clang-tidy.sh into two scripts (one for build, and one for run) because keeping it all in one file was getting cumbersome.
This also led to quite a bit of pruning of what seems to be legacy code in the script and the workflow, which was used for active development of the plugin back in the day. But in the current state (even without this PR) I find it hard to believe that anyone actually runs ./build-scripts/clang-tidy.sh for local development given the amount of effort needed to do so (and the sparse anecdotal experience of people in discord).

As a drive-by change, I bumped the github runner to ubuntu-24.04 which allowed me to stop adding custom llvm apt repo for clang-17, and instead reuse the stock ubuntu one (since ubuntu-24.04 has clang-17 in stock)

Describe alternatives you've considered

• Genuinely not doing this. I don't think this PR makes things worse, but it might be hard to review, and the savings are on the order of 10 cpu-minutes per run, which is not that much? Maybe? But I haven't actually looked into github action economy, and perhaps those 10-minutes-per-run add up..

Testing

Seems to work in my local fork moxian#4
Let's see if it also works in this upstream CI

Additional context