fix(ci): add permissions for website generation

Author: blackheaven

.github/workflows/nix.yml

@@ -34,6 +34,8 @@ jobs:
     if: ${{ github.ref == 'refs/heads/main' }}
     needs: check-advisories
     runs-on: ubuntu-22.04
+    permissions:
+      contents: write # for git push (s0/git-publish-subdir-action)
     steps:
       - uses: actions/checkout@v4
         with: