CI: restore coverlet with configured package sources (#1909)

* CI: defer coverlet restore to solution build

* CI: pin coverlet msbuild version

Author: Jim8y

.github/workflows/main.yml

@@ -38,7 +38,7 @@ jobs:
         path: ${{ env.NUGET_PACKAGES }}
         key: ${{ runner.os }}-dotnet-${{ env.DOTNET_VERSION }}-nuget-${{ hashFiles('NuGet.Config', 'src/Directory.Build.props', 'tests/Directory.Build.props', '**/*.csproj') }}
     - name: Add package coverlet.msbuild
-      run: find tests -path '*UnitTests/*.csproj' | xargs -I % dotnet add % package coverlet.msbuild --source https://api.nuget.org/v3/index.json
+      run: find tests -path '*UnitTests/*.csproj' | xargs -I % dotnet add % package coverlet.msbuild --version 6.0.4 --no-restore
     - name: Build Solution
       run: dotnet build ./neo-devpack-dotnet.sln
     - name: Check format