fix: resolve duplicate authorization header in workflow

Add persist-credentials: false to checkout action to prevent credential
conflicts with create-pull-request action.

Fixes: https://github.com/cachix/nixpkgs-python/actions/runs/19552890000/job/55988580724

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: domenkozar

.github/workflows/update.yml

@@ -16,6 +16,7 @@ jobs:
       - uses: actions/checkout@v6
         with:
           ref: main
+          persist-credentials: false
       - name: Free Disk Space (Ubuntu)
         uses: jlumbroso/free-disk-space@main
       - name: Install Nix