add package source to yml

.github/workflows/build.yml

@@ -20,6 +20,8 @@ jobs:
       uses: actions/setup-dotnet@v4
       with:
         dotnet-version: 8.0.x
+    - name: Add nuget package source
+      run: dotnet nuget add source --username USERNAME --password ${{ secrets.GITHUB_TOKEN }} --store-password-in-clear-text --name github "https://nuget.pkg.github.com/DFE-Digital/index.json"
     - name: Restore dependencies
       run: dotnet restore
     - name: Build