[infra] Switch mac deterministic dbg builder to use OS 10.15

Bug: 1122771
Change-Id: Icd0922c3dc77aaa175dd81cc753033490e2b71cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380815
Auto-Submit: Gregory Guterman <guterman@google.com>
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802433}

infra/config/generated/cr-buildbucket.cfg

@@ -5388,7 +5388,7 @@ buckets {
       swarming_tags: "vpython:native-python-wrapper"
       dimensions: "builder:Mac deterministic (dbg)"
       dimensions: "cpu:x86-64"
-      dimensions: "os:Mac-10.13"
+      dimensions: "os:Mac-10.15"
       dimensions: "pool:luci.chromium.ci"
       exe {
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"

infra/config/subprojects/chromium/ci.star

@@ -2899,6 +2899,7 @@ ci.fyi_mac_builder(
     cores = None,
     executable = "recipe:swarming/deterministic_build",
     execution_timeout = 6 * time.hour,
+    os = os.MAC_10_15,
 )
 
 ci.fyi_mac_builder(