ci: assigner: fix the main operation selector check

Fix an "if" that should have been an "elif". This is currently causing
the script to fallback into the "do all unassigned PRs of the day" path
when a PR is specified.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

scripts/set_assignees.py

@@ -327,7 +327,7 @@ def main():
 
     if args.pull_request:
         process_pr(gh, maintainer_file, args.pull_request)
-    if args.issue:
+    elif args.issue:
         process_issue(gh, maintainer_file, args.issue)
     elif args.modules:
         process_modules(gh, maintainer_file)