bcr-pr-reviewer: notify BCR maintainer if the PR author is also the module maintainer (#2175)

In this case, the author themself cannot review the PR.

Address PRs like
https://github.com/bazelbuild/bazel-central-registry/pull/3721 being
ignored.
diff --git a/actions/bcr-pr-reviewer/index.js b/actions/bcr-pr-reviewer/index.js
index 981cc31..de2b584 100644
--- a/actions/bcr-pr-reviewer/index.js
+++ b/actions/bcr-pr-reviewer/index.js
@@ -104,6 +104,10 @@
       maintainersCopy.delete(`@${prAuthor}`);
     }
     if (maintainersCopy.size === 0) {
+      // If all maintainers are skipped, we should notify the BCR maintainers
+      await postComment(octokit, owner, repo, prNumber,
+        `Hello @bazelbuild/bcr-maintainers, modules (${modulesList}) have been updated in this PR.
+        Please review the changes. You can view a diff against the previous version in the "Generate module diff" check.`);
       continue;
     }
     const maintainersList = Array.from(maintainersCopy).join(', ');