Move permissions in build workflow to top-level

To align with the CI best practices: https://llvm.org/docs/CIBestPractices.html#top-level-read-permissions

Author: lukel97

.github/workflows/build.yaml

@@ -4,12 +4,14 @@ on:
   push:
     branches: ['main']
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ubuntu-latest
 
     permissions:
-      contents: read
       packages: write
 
     steps: