Update CI workflow to set working directory for backend

Author: detilium

.github/workflows/ci.yml

@@ -22,7 +22,10 @@ jobs:
         dotnet-version: 10.0.x
     - name: Restore dependencies
       run: dotnet restore
+      working-directory: src/backend
     - name: Build
       run: dotnet build --no-restore
+      working-directory: src/backend
     - name: Test
       run: dotnet test --no-build --verbosity normal
+      working-directory: src/backend