Compare case-insenitively for full cache key match

[kbriggs]

Some self-hosted (or otherwise non-github-hosted) runners are non-case-preserving for cache keys. To support such configurations, set the cache-hit output variable to true if the found cache key matches case-insenitively.

Fixes: #302

[kbriggs]

Testing

With the change, with a codeberg action using my branch:

Cache Configuration
Cache Provider:
    github
…
Restore Key:
    v0-rust-cargodeps-Linux-x64-345d6b90
Cache Key:
    v0-rust-cargodeps-Linux-x64-345d6b90-10ea9ea9
.. Prefix:
  - v0-rust-cargodeps-Linux-x64
…
... Restoring cache ...
Cache Size: ~155 MB (162100487 B)
[command]/usr/bin/tar -xf /tmp/2b6d26c1-89f4-4f44-875f-cfcda9061c05/cache.tzst -P -C /workspace/kbriggs/airtouch5 --use-compress-program unzstd
Cache restored successfully
Restored from cache key "v0-rust-cargodeps-linux-x64-345d6b90-10ea9ea9" full match: true.

(and the subsequent cargo fetch step is correctly skipped, since cache-hit is now true)