Add NuGet source for TimetableDesigner

.github/workflows/pull_request_main.yml

@@ -18,5 +18,7 @@ jobs:
         uses: actions/setup-dotnet@v3
         with:
           dotnet-version: 10.0.x
+      - name: Add source
+        run: dotnet nuget add source --username TimetableDesigner --password ${{secrets.GITHUB_TOKEN }} --store-password-in-clear-text --name github "https://nuget.pkg.github.com/TimetableDesigner/index.json"
       - name: Build
         run: dotnet build