ci: try running on older versions with macos instead of ubuntu

Author: LinqLover

.github/workflows/develop.yml

@@ -17,14 +17,14 @@ jobs:
       fail-fast: false
       matrix:
         # Select platform(s)
-        os: [ ubuntu-latest ]
+        os: [ macos-latest ]
         # Select compatible Smalltalk image(s)
         smalltalk:
           - Squeak64-trunk
           - Squeak64-6.0
           # temporarily disabled because vm does not work on current ubuntu??
-          #- Squeak64-5.3
-          #- Squeak64-5.2
+          - Squeak64-5.3
+          - Squeak64-5.2
     name: ${{ matrix.smalltalk }} on ${{ matrix.os }}
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it