ci: add checkout step to auto-assign workflow

Included `actions/checkout@v4` to ensure the repository code is
available during workflow execution.

Author: shorodilov

.github/workflows/auto-assign.yml

@@ -12,6 +12,7 @@ jobs:
   assign:
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v4
       - uses: kentaro-m/[email protected]
         with:
           repo-token: "${{ secrets.GITHUB_TOKEN }}"