update_stats workflow: set up and use actions cache

.github/workflows/update_stats.yml

@@ -30,8 +30,29 @@ jobs:
           pip install setuptools --upgrade
           pip install .
 
+      # cache key syntax: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/caching-dependencies-to-speed-up-workflows#using-contexts-to-create-cache-keys
+      - name: Restore cached files
+        id: cache-files-restore
+        uses: actions/cache/restore@v4
+        with: 
+          path: ./cache
+          key: ${{ runner.os }}-${{ hashFiles('~/.cache_key') }}
 
       - name: Run update script
         run: |
           python -m repo_stats.runner -a ${{ secrets.ADS_TOKEN }} -g ${{ secrets.GIT_TOKEN }} -c "cache" 
 
+      # caches are immutable, so create a new one (new key) each time any of the files being cached change.
+      # the cache key will be new if any files in the cache path have been changed by 'repo_stats.runner'.
+      # otherwise the cache save will 'fail' (because there's no need to update it - that's fine)
+      - name: Update cached files
+        id: cache-files-save
+        uses: actions/cache/save@v4
+        with:
+          path: ./cache
+          key: ${{ runner.os }}-${{ hashFiles('./cache') }}
+
+      - name: Save cache key 
+        run: |
+          echo ${{ hashFiles('./cache') }} >> ~/.cache_key
+