Build: run macOS workflows on macos-10.15 (Catalina)

Author: Oldes

.github/workflows/main.yml

@@ -94,7 +94,7 @@ jobs:
           path: ./rebol3-*.gz
 
   macos:
-    runs-on: macos-latest
+    runs-on: macos-10.15
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2