Enabled cloning of submodules in migrations test

- we start Ghost in this test but Ghost will check Casper, which isn't
  cloned without this commit

.github/workflows/test.yml

@@ -43,6 +43,9 @@ jobs:
     name: Migrations - ${{ matrix.DB }}
     steps:
       - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+          submodules: true
       - uses: actions/setup-node@v1
         with:
           node-version: '12.10.0'