Force RT's global.json on SS14 in content test

So that it uses the right .NET SDK version when we update like this.

.github/workflows/test-content.yml

@@ -30,6 +30,8 @@ jobs:
           git fetch origin ${{ github.sha }}
           git checkout FETCH_HEAD
           git submodule update --init --recursive
+      - name: Replace global.json
+        run: cp RobustToolbox/global.json .
       - name: Install dependencies
         run: dotnet restore
       - name: Build