feat(cache-magento): include runner.os in the cache key (#245)

Composer's download cache contains platform-specific binaries and
extracted archives that aren't safe to share across operating
systems. Add the runner OS as a key segment so a Linux job won't
restore a macOS-built cache (or vice versa).

cache-magento/action.yml

@@ -37,6 +37,7 @@ runs:
       run: |
         bash "${{ github.action_path }}/compute-cache-keys.sh" \
           "${{ inputs.composer_cache_key }}" \
+          "${{ runner.os }}" \
           "${{ steps.cache-magento-get-php-version.outputs.version }}" \
           "${{ steps.cache-magento-get-composer-version.outputs.version }}" \
           >> $GITHUB_OUTPUT