Speed up macOS actions by using larger instance (#624)

Summary:
Pull Request resolved: #624

Switch to managed macos-12-xl to speed up macos unittests.

Reviewed By: d4l3k

Differential Revision: D40562764

fbshipit-source-id: fdbe1a06cd95b244018e55550d103e840e1844b2

Author: kurman

.github/workflows/python-unittests.yaml

@@ -14,7 +14,7 @@ jobs:
         platform: [ubuntu-18.04]
         include:
           - python-version: 3.9
-            platform: macos-latest
+            platform: macos-12-xl
       fail-fast: false
     runs-on: ${{ matrix.platform }}
     steps: