Update CI workflow to include Pharo64-12

Author: alesshosry

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        smalltalk: [ Pharo64-13, Pharo64-14 ]
+        smalltalk: [ Pharo64-12, Pharo64-13, Pharo64-14 ]
     name: ${{ matrix.smalltalk }}
     steps:
       - uses: actions/checkout@v4