Allow updating/overwriting the matching cache

[jjlin]

If I cache something that can be incrementally updated, it would be nice to be able to just overwrite the existing cache with the updated copy. For example, if my build involves cloning a large-ish external Git repo, I would like to do something like

- name: Restore repo from cache
  uses: actions/cache@v2
  with:
    path: ext-git-repo
    key: ext-git-repo
    overwrite-cache: true
- name: Update repo
  working-directory: ext-git-repo
  run: git fetch ...

This looks much cleaner (and is more space-efficient for GitHub) than the current alternative that generates many redundant copies of the cache:

- name: Restore repo from cache
  uses: actions/cache@v2
  with:
    path: ext-git-repo
    key: ext-git-repo-${{ github.run_number }}
    restore-keys: |
      ext-git-repo-

[Purpzie]

+1. Just wanted to add that this is especially relevant to rust, since the entire crates.io index, which is incrementally updated, usually needs to be available for every job or it'll take a long time to download.