Try back-dating MacOS version in pipeline for better support/fix breakage.

.github/workflows/main.yml

@@ -8,7 +8,7 @@ on:
       - test
 jobs:
   build-mac:
-    runs-on: macOS-latest
+    runs-on: macos-10.15
     steps:
     - uses: actions/checkout@v2
     - name: Install Qt 5.15.2