fix env slug

Author: kaeizen

.github/workflows/playwright.yml

@@ -34,7 +34,7 @@ jobs:
           WP_AUTH_STORAGE: wp-auth.json
           WP_USERNAME: admin
           WP_PASSWORD: password
-          STACKABLE_SLUG: stackable-$VERSION_SUFFIX
+          STACKABLE_SLUG: stackable-${{ env.VERSION_SUFFIX }}
       run: npm run test
     - uses: actions/upload-artifact@v4
       if: ${{ !cancelled() }}