workflow: When updating the issueXX branch, use force push

[thieta]

Summary

Otherwise if you try to update the branch with a new /cherry-pick
from the same issue you will run into problems similar as to the
one shown in this workflow:
https://github.com/llvm/llvm-project/runs/5864672298?check_suite_focus=true

[tstellar]

This looks good, thank you.

llvm/utils/git/github-automation.py

def create_branch(self, commits:List[str]) -> bool:
             try:
                 local_repo.git.cherry_pick('-x', c)
             except Exception as e:
                 self.issue_notify_cherry_pick_failure(c)
                 raise e
 
         push_url = self.push_url
         print('Pushing to {} {}'.format(push_url, branch_name))
-        local_repo.git.push(push_url, 'HEAD:{}'.format(branch_name))
+        local_repo.git.push(push_url, 'HEAD:{}'.format(branch_name), force=True)
 
         self.issue_notify_branch()
         self.issue_remove_cherry_pick_failed_label()
         return True
 
 
     def create_pull_request(self, owner:str, branch:str) -> bool:
         """