Webpack bundle size analysis: Use Job ID

With a recent update [0] to the bundle analysis we can provide the Job
ID of our current pipeline which will allow us to retrieve the correct
commits more reliably and link to the artifacts of the build job.

[0]: https://gitlab.com/gitlab-org/frontend/playground/webpack-memory-metrics/-/merge_requests/11

danger/bundle_size/Dangerfile

@@ -22,7 +22,7 @@ analyze_cmd = [
 # from master that was merged into for the merged pipeline.
 comparison_cmd = [
   "webpack-compare-reports",
-  "--from-sha #{gitlab.mr_json["diff_refs"]["start_sha"]}",
+  "--job #{ENV["CI_JOB_ID"]}",
   "--to-file #{analysis_result}",
   "--html ./bundle-size-review/comparison.html",
   "--markdown #{markdown_result}"