Add CELab builder CI trigger.

Bot config was added in https://crrev.com/c/1649302 but our recipe and tests
didn't work yet so we disabled the trigger in https://crrev.com/c/1656969.

They should now work (tested with our try bot), so restoring the trigger.

Change-Id: Iddec92e74ad9d3d608f6e5fdc05b2f58fde1d576
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670607
Auto-Submit: Mathieu Binette <mbinette@google.com>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671737}

infra/config/luci-scheduler.cfg

@@ -374,6 +374,7 @@ trigger {
   triggers: "win-archive-dbg"
   triggers: "win-jumbo-rel"
   triggers: "win-archive-rel"
+  triggers: "win-celab-builder-rel"
   triggers: "win-pixel-builder-rel"
   triggers: "win32-arm64-rel"
   triggers: "win32-archive-dbg"