OWNERS: grant copybara access to repo_launcher

We're using copybara to sync upstream repo here automatically.
Grant it owners access so more devs can help merge its syncs.

Change-Id: I54acf18c73af6e8c8a670d2761ad60b2a778a20a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6419683
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

OWNERS

@@ -52,6 +52,7 @@ per-file cros_sdk=file://CROS_OWNERS
 per-file gerrit=file://CROS_OWNERS
 per-file repo=file://CROS_OWNERS
 per-file repo_launcher=file://CROS_OWNERS
+per-file repo_launcher=copybara-worker-blackhole@google.com #{LAST_RESORT_SUGGESTION}
 
 # LUCI related tooling
 per-file bb*=file://LUCI_OWNERS