Check action bundles buf correctly (#190)

This PR ensures CI runs the action built from this commit. We output the
buf version from the given action to check the buf cli is successfully
fetched.

.github/workflows/ci.yaml

@@ -28,3 +28,6 @@ jobs:
         run: make generate && make checkgenerate
       - name: Build
         run: make build && make checkgenerate
+      - name: Validate Action
+        uses: ./
+      - run: buf --version