Revert "removed duplicated line into main.yml"

This reverts commit e81f95b.

Apparently it's needed by CI

.github/workflows/main.yml

@@ -16,6 +16,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      - uses: actions/checkout@v4
       - uses: actions/checkout@v4
         with:
           ref: gh-pages