Cache pixi.lock after its creation

[christophfroehlich]

Fix to #391
two errors:

Error: You cannot specify cache-write = true without a lock file present

and

Run actions/cache/save@v4
Warning: Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.
Warning: Cache save failed.

because it cache/save was on the wrong place