Check generated code is current from ci.

Rather than committing from ci, print an error if the generated code is out of
date. I think reviewing the generated code is an important part of the
development process, so would expect the developer to do this before
committing.

Author: jmcarp

.github/workflows/build-test.yml

@@ -21,3 +21,11 @@ jobs:
         shell: bash
         run: |
           make lint
+      - name: generate
+        shell: bash
+        run: |
+          make generate
+          if ! git diff --exit-code; then
+            echo 'Generated files are out of date. Run `make generate` and commit the result.'
+            exit 1
+          fi