ci: declare contents: read on zig compiler workflow

zig.yml runs the Zig cross-compile of aws-lc on multiple architectures.
Pure CI - checkout + build + test. No git pushes or comments.

Author: arpitjain099

.github/workflows/zig.yml

@@ -7,6 +7,10 @@ on:
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref_name }}
   cancel-in-progress: true
+
+permissions:
+  contents: read
+
 jobs:
   zig:
     if: github.repository_owner == 'aws'