build: pin macOS GitHub runner to macos-10.15

Node.js 12.x requires Python 2 to build. `macos-latest` GitHub
hosted runners are now macOS 11 and do not have Python 2 available.

PR-URL: #41124
Refs: actions/runner-images#4060
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

.github/workflows/test-macos.yml

@@ -8,11 +8,11 @@ env:
 
 jobs:
   test-macOS:
-    runs-on: macos-latest
+    runs-on: "macos-10.15"
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ env.PYTHON_VERSION }}
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v2
         with:
           python-version: ${{ env.PYTHON_VERSION }}
       - name: Environment Information