ci: add checkout step for tests

Author: rcasia

.github/workflows/makefile.yml

@@ -13,6 +13,9 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      - name: checkout
+        uses: actions/checkout@v4
+
       - uses: rhysd/action-setup-vim@v1
         with:
           neovim: true