TRAP Caching: Disable on self-hosted runners

[edoardopirovano]

Self-hosted runners may have slow network connections, which means they are unlikely to benefit from TRAP caching. So, let's disable this feature by default on self-hosted runners (whilst still respecting what the workflow says if the user has explicitly enabled/disabled it there).

Checking of whether we're on a self-hosted runner is rather awkward. I was hoping there would just be an environment variable that tells us this, but sadly there is not. Instead, I've implemented a heuristic on the runner name which checks against the two currently possible names of GitHub-hosted runners. As a fallback, we also look for a toolcache called hostedtoolcache which is what the folder is always called on GitHub-hosted runners. Hopefully the fact we're checking two things makes this somewhat robust, although it's still more brittle than I would like and I am definitely open to a suggestion on a better solution.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.