Grant workflow write permissions.

Author: ScottTodd

.github/workflows/update_iree_requirement_pins.yml

@@ -12,6 +12,10 @@ on:
     # Weekdays at 11:00 AM UTC = 03:00 AM PST / 04:00 AM PDT
     - cron: "0 11 * * 1-5"
 
+permissions:
+  contents: write
+  pull-requests: write
+
 jobs:
   update-iree:
     if: ${{ github.repository_owner == 'nod-ai' || github.event_name != 'schedule' }}