Fix .NET 6 CI false positives

OpenRA PR 21769.

Author: penev92

.github/workflows/ci.yml

@@ -13,6 +13,9 @@ jobs:
     runs-on: ubuntu-22.04
 
     steps:
+      - name: Remove System .NET
+        run: sudo apt-get remove -y dotnet*
+
       - name: Clone Repository
         uses: actions/checkout@v4