Only cache changed NuGet and NPM packages when using custom images (OSOE-711)

[Piedone]

Once github/roadmap#826 will be available, we'll use custom VM images to pre-cache something like a typical Orchard Core solution's NuGet and NPM packages. Then, build-dotnet's NuGet and NPM caching will be less useful, because it'll try to always cache all packages, even the ones coming from the image.

Thus, modify the caching logic to only cache those packages that changed (were added) in the current workflow run, not those that come with the image.

Jira issue