Reverts zed-industries/zed#48607
The problem we've ran into is that CI started reusing already-cached
runs from newer branches (that did not have that change pulled in).
Yet this unveiled another potential issue, which is that relying on
mtime might make it so that an artifact for a different (newer) commit
would be reused by an unrelated run of CI.
The answer is: checksum-based-freshness.
Release Notes:
- N/A