Fix unused sorted value in update-release-branch

Fix a minor issue in the update-release-branch.py script that performs a
call to `sorted` but doesn't use the output. Since `sorted` does not
operate in place, the call is currently useless. As a result, the function
`get_pr_for_commit` does not currently work as exected. I.e. it is
expected to return the "first" (i.e. lowest PR number), but actually it
returns the first in the list provided by GitHub.

.github/update-release-branch.py

@@ -123,8 +123,8 @@ def get_pr_for_commit(repo, commit):
   if prs.totalCount > 0:
     # In the case that there are multiple PRs, return the earliest one
     prs = list(prs)
-    sorted(prs, key=lambda pr: int(pr.number))
-    return prs[0]
+    sorted_prs = sorted(prs, key=lambda pr: int(pr.number))
+    return sorted_prs[0]
   else:
     return None