Add continue-on-error for cache step, and upgrade GH cache action

[rnorth]

... to work around apparent problems with GitHub caching of the (post-build) cache push step.
e.g.

Post job cleanup.
/bin/tar -cz -f /home/runner/work/_temp/daec269c-22ea-41a1-a4eb-ede3c9cf30f1/cache.tgz -C /home/runner/.gradle/caches .
[warning]Cache service responded with 503 during chunk upload.
events.js:187
      throw er; // Unhandled 'error' event
      ^

Error: ESPIPE: invalid seek, read
Emitted 'error' event on ReadStream instance at:
    at internal/fs/streams.js:167:12
    at FSReqCallback.wrapper [as oncomplete] (fs.js:470:5) {
  errno: -29,
  code: 'ESPIPE',
  syscall: 'read'
}

This failure is obviously a problem with the caching system, and should not fail our build.