fix(github): add id-token: write permission to purge CDN workflow (#4486)

The `_purge_cdn.yaml` workflow had `permissions: {}` which removed all
permissions, preventing Azure federated credential login from obtaining
the OIDC token needed for authentication.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: pomfrida

.github/workflows/_purge_cdn.yaml

@@ -11,7 +11,8 @@ on:
 jobs:
   purge:
     name: Purge the cdn to refresh cache
-    permissions: {}
+    permissions:
+      id-token: write
     runs-on: ubuntu-latest
     environment: ${{ inputs.environment }}
     env: