Don't run GHA on push; cancel in progress runs when new code pushed.

Author: YPCrumble

.github/workflows/molecule.yml

@@ -1,14 +1,17 @@
 ---
 name: Molecule Test
 on:
-  push:
   pull_request:
   schedule:
     - cron: '30 5 * * 0'
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
   build:
     runs-on: ubuntu-latest
     strategy:
+      fail-fast: true
       max-parallel: 4
       matrix:
         python-version: ["3.12", "3.13"]