Make workflow pull in engine changes required by a PR (#1123)

Using a fancy new GitHub action, this makes the workflow prevent
automatic submodule updates and allows for pulling engine changes in
from a required engine PR.

.github/workflows/build-test.yml

@@ -14,7 +14,11 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Setup submodule
-      run: git submodule update --init --recursive
+      run: |
+        git submodule update --init --recursive
+        touch BuildChecker/DISABLE_SUBMODULE_AUTOUPDATE
+    - name: Pull engine updates
+      uses: ivanbakel/submodule-dependency@v0.1.0
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with: