Remove top-level permissions from PR workflow

Top-level permissions on workflow_call workflows act as
a ceiling that blocks caller-granted permissions, causing
startup_failure. Use per-job permissions only.

Author: bschwedler

.github/workflows/bakery-build-pr.yml

@@ -52,8 +52,6 @@ on:
         type: string
     # NO secrets section — only inherited GITHUB_TOKEN
 
-permissions: {}
-
 defaults:
   run:
     shell: bash