Use persistent checkout for UpdateMetaConfig bot

Bot is failing without this.

BUG=skia:6591

Change-Id: I29cf5e18a7fa240d3e45bdadf83c1f91060f45f5
Reviewed-on: https://skia-review.googlesource.com/17319
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
5 files changed