Useful for custom forks of Godot which don't want to run our CI for any
reason.
This is configured in `settings/variables/actions` for the repository,
setting it to any value aside from an empty string will skip all jobs.
This ensures that the godot-cpp job has plenty of resources
to run its build and avoid being affected by the main build.
Additionally:
- Extract test tasks into dedicated actions.
- Upload artifacts as early as possible.
- Ensure that we check master cache before random cache.
- file_format, header_guards and clang-format benefit from this short list.
- dotnet-format, Python and JS checks don't, but they're only relevant for
PRs changing a specific set of files, so we skip them when those files
aren't modified.
The logic to get changed files only works reliably for:
- Pull request events
- Non-force pushed push events
So when force pushing a branch in your fork, or creating a new branch,
it will still scan all files as fallback.
Upgraded CI runner to Ubuntu 22.04 so we get clang-format 14 out of the box,
so we don't need to install a custom version (saves ~15 s). We also cache
the APT dependencies to speed up the build and avoid flaky Ubuntu/Microsoft
repos.
