ci: update presubmit validation (gemini-cli-extensions#74)

Author: averikitsch

.github/workflows/presubmit-tests.yml

@@ -34,4 +34,6 @@ jobs:
           chmod +x toolbox
 
       - name: Install Extension
-        run: yes | npx gemini extensions install .
+        run: npx gemini extensions validate .
+        env:
+          GEMINI_API_KEY: "placeholder"