`Relative pathing '.' ... is not allowed` discriminates against local actions

#684 asked about relative paths, and was rejected because there were other things someone could do...

I'm developing an action and I'd like to cache content within my action. Most of the time, my action will be run from something like: /home/runner/work/check-spelling/check-spelling/, but when it's testing itself, the github.action_path property has a trailing ./. Afaict, GHA does not have string processing functions that would make stripping a trailing ./ or / easy to do.

The result is:
https://github.com/jsoref/check-spelling/actions/runs/3993931770/jobs/6851091696

Run actions/cache@v3
  with:
    path: /home/runner/work/check-spelling/check-spelling/.//dictionaries
    key: check-spelling-dictionaries-
    enableCrossOsArchive: false
  env:
    workflow_check_commit_messages: commits
    PATH: /home/runner/work/check-spelling/check-spelling/.//wrappers:/home/runner/.local/bin:/opt/pipx_bin:/home/runner/.cargo/bin:/home/runner/.config/composer/vendor/bin:/usr/local/.ghcup/bin:/home/runner/.dotnet/tools:/snap/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
    replace_v3_actions_checkout: 1
Cache not found for input keys: check-spelling-dictionaries-

followed by

Post job cleanup.
Post job cleanup.
Warning: Invalid pattern '/home/runner/work/check-spelling/check-spelling/.//dictionaries'. Relative pathing '.' and '..' is not allowed.

Rejecting '..' is perfectly reasonable.
Rejecting /./ seems questionable, especially given that GHA naturally constructs paths of this form.

If GHA were to not construct paths of this form (which would make me otherwise happy), I'd be happy to close this Issue...

p.s. it would be nicer if the complaint about "relative pathing" was triggered in the normal uses stage instead of in the post stage.

Start by reproducing the reported workflow with actions/cache@v3 and inspect the warning produced during post-job cleanup. Trace how the cache action handles the logged path /home/runner/work/check-spelling/check-spelling/.//dictionaries; done means /./ is accepted while .. remains rejected, with any validation warning occurring at the expected stage.