Enable submodules for NumPy workflow

Author: honno

Diff for: .github/workflows/numpy.yml

@@ -11,7 +11,10 @@ jobs:
         python-version: [3.8, 3.9]
 
     steps:
-    - uses: actions/checkout@v1
+    - name: Checkout array-api-tests
+      uses: actions/checkout@v1
+      with:
+        submodules: 'true'
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v1
       with: