[sk8s] Force refresh of access token.
[sk8s] Force refresh of access token.

Do that by creating a new TokenSource on every interval.
See https://github.com/golang/oauth2/pull/396 for more details.

Change-Id: I0cbf80c0b6d9dc5b21348fd1aab14de67876ca52
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/288044
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot