Temporary fix github checkout action.

Temporary: before migrating pypi build/publish to each submodules.

Author: TontonSancho

.github/workflows/publish-on-pypi.yml

@@ -20,7 +20,10 @@ jobs:
     if: github.event_name == 'push' || github.event_name == 'release'
 
     steps:
-    - uses: actions/checkout@v4
+    - name: Checkout
+      uses: actions/checkout@v4
+      with:
+        submodules: 'true'
     - name: Set up Python
       uses: actions/setup-python@v5
       with: