ci: Check that build steps didn’t change any files (#5373)

Author: edemaine

.github/workflows/continuous-integration.yml

@@ -43,3 +43,6 @@ jobs:
     - run: node ./bin/cake build:parser
     - run: node ./bin/cake build:full
     - run: node ./bin/cake build:browser
+
+    - run: git diff --exit-code
+      if: matrix.node-version != '6.x' && matrix.node-version != '8.x'