Bug 1547044. Properly set windows10-aarch64 tests to tier 2. r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D28887

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/talos.yml

@@ -4,7 +4,7 @@ job-defaults:
     tier:
         by-test-platform:
             windows10-64-ccov/.*: 3
-            windows10-aarch64/.*: 3
+            windows10-aarch64/.*: 2
             linux64-ccov/.*: 3
             default: default
     virtualization: