infra: Switch mac-rel on the CQ to builderfull.

crrev.com/i/3462032 allocated 100 machines for this pool:
http://shortn/_z6nSkdF9st

That should be about enough according to http://shortn/_PPwEhxY2rU,
especially since cache hit-ratios should be much better now after this.

Bug: 1140727
Change-Id: Ic48d4f7fccde6aca434ffb1d7e1fc0f3381d757c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587558Reviewed-by: Haiyang Pan <hypan@google.com>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836320}

infra/config/generated/cr-buildbucket.cfg

@@ -37504,7 +37504,7 @@ buckets {
       name: "mac-rel"
       swarming_host: "chromium-swarm.appspot.com"
       swarming_tags: "vpython:native-python-wrapper"
-      dimensions: "builderless:1"
+      dimensions: "builder:mac-rel"
       dimensions: "cpu:x86-64"
       dimensions: "os:Mac-10.15"
       dimensions: "pool:luci.chromium.try"

infra/config/subprojects/chromium/try.star

@@ -1292,6 +1292,7 @@ try_.chromium_mac_builder(
 try_.chromium_mac_builder(
     name = "mac-rel",
     branch_selector = branches.STANDARD_MILESTONE,
+    builderless = not settings.is_master,
     use_clang_coverage = True,
     goma_jobs = goma.jobs.J150,
     main_list_view = "try",