update dev workflow to use ubuntu-latest for build-test job

Co-authored-by: Copilot <copilot@github.com>

.gitea/workflows/dev.yml

@@ -8,11 +8,16 @@ on:
 
 jobs:
   build-test:
-    runs-on: ubuntu-dotnet
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v4
 
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: '9.0.x'
+
       - name: Restore backend
         run: dotnet restore