CI: Checkout ppm submodule for package tests too

Again so that the "yarn dist" command won't warn and exit.

.github/workflows/package-tests-linux.yml

@@ -16,6 +16,8 @@ jobs:
     steps:
     - name: Checkout the latest code
       uses: actions/checkout@v4
+      with:
+        submodules: true
 
     - name: Setup node
       uses: actions/setup-node@v4