nupkg files require all architectures even with selective builds

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.

Start by comparing the packaging files NuGet/chromiumembeddedframework.runtime.json and NuGet/chromiumembeddedframework.runtime.nuspec with the linked CefSharp.Common.NETCore.targets section. Trace how selective architecture builds affect package restore and manual copies. Done means an x64-only build no longer requires or attempts to restore arm64 binaries.

Written by the indexing model from the issue text.