ci(setup-install): adopt cache-magento stamp cache (#249)

Adds `composer update --no-install` ahead of cache-magento so the
lock exists when the stamp key is computed, then flips on
stamp:true with the matching working-directory.

.github/workflows/_internal-setup-install.yaml

@@ -54,10 +54,15 @@ jobs:
         magento_version: ${{ matrix.magento }}
         magento_repository: "https://mirror.mage-os.org/"
         composer_auth: ${{ secrets.COMPOSER_AUTH }}
+    
+    - run: composer update --no-install
+      working-directory: ${{ steps.setup-magento.outputs.path }}
 
     - uses: ./cache-magento
       with:
         composer_cache_key: ${{ matrix.magento }}
+        working-directory: ${{ steps.setup-magento.outputs.path }}
+        stamp: true
 
     - name: Add extension repository
       working-directory: ${{ steps.setup-magento.outputs.path }}