Add explicit permissions block to build-single-toolchain workflow for security

Co-authored-by: BernardXiong <1241087+BernardXiong@users.noreply.github.com>

.github/workflows/build-single-toolchain.yml

@@ -18,6 +18,9 @@ on:
         type: boolean
         default: true
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ubuntu-latest