ci: verify JS build artifacts aren't committed

Building our JS packages dirties the repo. This makes sure we don't
accidentally commit these to the repo, as it'll break Flow tests with an
obscure error message.

Changelog: [Internal]

Testing:

Ran this locally, the GH lint action should run on this PR.
I'll intentionally add a build artifact to validate.

.github/actions/lint/action.yml

@@ -30,6 +30,9 @@ runs:
     - name: Lint java
       shell: bash
       run: ./scripts/circleci/exec_swallow_error.sh yarn lint-java --check
+    - name: Verify not committing repo after running build
+      shell: bash
+      run: yarn run build --check
     - name: Run flowcheck
       shell: bash
       run: yarn flow-check