[GitHub] Run format job on stacked PRs (llvm#102957)

Currently the formatter only runs for the main branch, which prevents
the formatter from running for stacked PRs, which have to target user
branches instead of main.

Author: tobias-stadler

.github/workflows/pr-code-format.yml

@@ -7,6 +7,7 @@ on:
   pull_request:
     branches:
       - main
+      - 'users/**'
 
 jobs:
   code_formatter: