Add explicit permissions to coverage workflow

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

Author: Copilot

.github/workflows/coverage.yml

@@ -12,6 +12,8 @@ on:
 jobs:
   coverage:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable