Change actions/checkout ref field

Looks like something has changed that now requires this to be set differently

Author: adbridge

.github/workflows/main.yml

@@ -21,7 +21,7 @@ jobs:
           repository: ARMmbed/mbed-os-ci-scripts
           token: ${{ secrets.ISSUE_TRIAGING_PERMS }}
           path: scripts
-          ref: triage_test
+          ref: mbed-os-ci-scripts
 
       - name: Run the script
         run: |