Checkout submodules on clone.

.github/workflows/rust.yml

@@ -15,6 +15,8 @@ jobs:
     steps:
       - name: Checkout sources
         uses: actions/checkout@v2
+        with:
+          submodules: true
 
       - name: Install stable toolchain
         uses: actions-rs/toolchain@v1
@@ -52,6 +54,8 @@ jobs:
     steps:
       - name: Checkout sources
         uses: actions/checkout@v2
+        with:
+          submodules: true
 
       - name: Install stable toolchain
         uses: actions-rs/toolchain@v1
@@ -89,6 +93,8 @@ jobs:
     steps:
       - name: Checkout sources
         uses: actions/checkout@v2
+        with:
+          submodules: true
 
       - name: Install stable toolchain
         uses: actions-rs/toolchain@v1