nupkg files require all architectures even with selective builds

[mitchcapper]

While https://github.com/cefsharp/cef-binary/pull/100 initially covered this the final implementation either intentionally or not no longer templates the spec files.

This can cause it to try and restore arm64 binaries when only x64 packages are built. This doesn't require major changes but for arm64 removal for example modifying: https://github.com/cefsharp/cef-binary/blob/master/NuGet/chromiumembeddedframework.runtime.json?#L13-L17

and potentially: https://github.com/cefsharp/cef-binary/blob/master/NuGet/chromiumembeddedframework.runtime.nuspec?#L30

it requires some in the main project too (as otherwise the manual copies still try) but probably there straight conditionals would work: https://github.com/cefsharp/CefSharp/blob/05577b7bf3933a057d42dabe0dc915676f9463a8/NuGet/PackageReference/CefSharp.Common.NETCore.targets?#L204-L224

I can do another PR to either remove these lines when select architectures are specified or go back to a templating system or nothing if it isn't desired for general support of select building.