.github: add actionlint to lint GitHub Actions YAML

[ES-Alexander]

Summary

• Adds rhysd/actionlint for linting GitHub Actions YAML
• Fixes issues found by running it manually

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Description

@tridge mentioned adding this to RSync in the recent EU DevCall, so I took a stab at adding it to ArduPilot.
I've kept the bulk of the original Action, just moved it to ubuntu-slim, bumped the checkout version, and cleaned up the comments a bit. I've also tied the download script to a commit hash, since otherwise it feels needlessly susceptible to a supply-chain attack (for reference, there is no official Action we can just "use" instead).

I ran it locally and found:

1. ./ local paths are apparently not supported by GitHub's Actions glob/path handling
   • I fixed a bunch of cases where that pattern was used
2. test_chibios was redundantly excluding a clang toolchain that's no longer used at all
   • I've removed that specifier, but if it's hoped to be added back at some point we could change to commenting that out instead
3. It was complaining about ardupilot-qurt (in qurt_build.yml) not being a standard runner
   • I assumed that exists as a self-hosted runner, and configured actionlint accordingly, but if that's incorrect it can instead be removed from the workflow

[tridge]

Thanks!

[ES-Alexander]

I've just tied the download script to a specific commit - it didn't seem to make sense to try to get a specific version by blindly passing a parameter into a dynamically updating download script.