Check go.mod and go.sum on BuildKite

Since Dependabot doesn't update these files. The failure is treated as
a "soft" failure.

https://github.com/dependabot/feedback/issues/215

Signed-off-by: Kazuyoshi Kato <[email protected]>

Author: kzys

.buildkite/pipeline.yml

@@ -123,6 +123,12 @@ steps:
     soft_fail:
       - exit_status: "*"
 
+  - label: 'go mod tidy'
+    commands:
+      - 'go mod tidy'
+      - test -z "$(git status --porcelain)" && exit 0 || git --no-pager diff && echo -e '\ngo.mod and/or go.sum differ from committed, please run "go mod tidy" and commit the updated files.\n' && exit 42
+    soft_fail:
+      - exit_status: 42 # Dependabot doesn't modify go.mod and go.sum
 
   # This allows the cleanup step to always run, regardless of test failure
   - wait: ~