Revert "Infra: Use Mac 10.15 machines for mac-arm64-rel tryjob"

This reverts commit 4421e3c9.

Reason for revert: Going to try to make arm64 compile on 10.13

Original change's description:
> Infra: Use Mac 10.15 machines for mac-arm64-rel tryjob
> 
> Bug: 1106950
> Change-Id: I69a3a8df07f40d0cc24de3f85212e51d5945bd46
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350149
> Auto-Submit: Gary Tong <gatong@chromium.org>
> Reviewed-by: Erik Staab <estaab@chromium.org>
> Commit-Queue: Erik Staab <estaab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#796959}

TBR=estaab@chromium.org,gbeaty@chromium.org,gatong@chromium.org

Change-Id: I3a6e9f78e3990113d3bfb059c0d2977e9d04fbac
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1106950
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350141Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Gary Tong <gatong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796977}

infra/config/generated/cr-buildbucket.cfg

@@ -26831,7 +26831,7 @@ buckets {
       swarming_tags: "vpython:native-python-wrapper"
       dimensions: "builderless:1"
       dimensions: "cpu:x86-64"
-      dimensions: "os:Mac-10.15"
+      dimensions: "os:Mac-10.13"
       dimensions: "pool:luci.chromium.try"
       dimensions: "ssd:1"
       exe {

infra/config/subprojects/chromium/try.star

@@ -479,7 +479,7 @@ try_.chromium_mac_builder(
 try_.chromium_mac_builder(
     name = "mac-arm64-rel",
     goma_jobs = goma.jobs.J150,
-    os = os.MAC_10_15,
+    os = os.MAC_10_13,
 )
 
 try_.chromium_mac_builder(