[BazelBot] Fix PR Creation

Tested.

We were using the wrong head branch and we need to disable maintainer
modification or otherwise we end up with failures from the Github API.

https://github.com/orgs/community/discussions/39178 has additional
information, but nothing conclusive.

Author: boomanaiden154

google-bazel-bot/bazel-bot/utils.py

@@ -287,8 +287,9 @@ def push_fix(self, commit_hash: str, create_pr: bool) -> bool:
             pr = self.gh_pr_repo.create_pull(
                 title=f"[Bazel] Fixes {commit_hash[:7]}",
                 body=pr_body,
-                head=f"{self.creds.gh_pr_user}:{branch_name}",
+                head=f"{self.creds.gh_fork_user}:{branch_name}",
                 base=self.main_branch,
+                maintainer_can_modify=False,
             )
             logger.info(f"Pull Request Created: {pr.html_url}")
         except Exception as e: