Is it possible to make the key optional and hence make the cache action read-only? This is in the case that you know that you only want to read from one of the "restore" keys and to fail if you can't find one.

For clarity, look for restore key patterns only:

On cache hit - read the cache, don't attempt to write a new one.
On cache miss - job fail.

An alternative could be to recognize a specific string as the key which means skip the write on job completion:

      uses: actions/cache@v1
      with:
        path: cache
        key: donotwrite
        restore-keys: myexpectedcache-