Use correct GHA workflow syntax in new cache-clearing action, be more resilient

[huonw]

In #20755, I had a typo: formatting the jobs as a list, with names keys... but that's the syntax for steps. Jobs need to be a dict with the ID as the key.

This also squashes any errors from rm -rf ....: on Mac, it seems that attempts to delete directories that the runner doesn't have permission for (particularly system ones in ~/Library/Caches). But that's fine, we'll still delete all of the caches created by "day-to-day" actions runs, which are the big ones.

(Why wasn't this caught in #20755? I attempted to run the workflow before merging, and got errors, but incorrectly assumed this was because the workflow didn't exist on main, but they were actually because it's invalid.)

This has now successfully run and cleared hundreds of gigs over two runs:

• https://github.com/pantsbuild/pants/actions/runs/8562624080 (required the "squash rm errors fix)
• https://github.com/pantsbuild/pants/actions/runs/8562947394/job/23467155450 (with that fix).