Use macOS 10.15 on GitHub Actions

This is in preparation for GitHub making macOS Big Sur the default at some point

Author: MaxDesiatov

.github/workflows/main.yml

@@ -109,7 +109,7 @@ jobs:
 
   macos_build:
     timeout-minutes: 0
-    runs-on: macos-latest
+    runs-on: macos-10.15
 
     steps:
       - uses: actions/checkout@v1
@@ -146,7 +146,7 @@ jobs:
 
   macos_smoke_test:
     name: Run smoke tests on macOS
-    runs-on: macos-latest
+    runs-on: macos-10.15
     needs: macos_build
     steps:
       - name: Download installable macOS archive