Generate nuget packages fails on non-Windows

[ViktorHofer]

When doing build.sh --pack to generate nuget packages in addition to building the repository, at least one nuspec fails to build.

/vmr/.dotnet/sdk/9.0.100-preview.2.24103.2/Sdks/NuGet.Build.Tasks.Pack/buildCrossTargeting/NuGet.Build.Tasks.Pack.targets(221,5): error NU5019: File not found: 'netcoreapp3.1\testhost.exe'. [/vmr/src/vstest/src/package/Microsoft.TestPlatform.TestHost/Microsoft.TestPlatform.TestHost.csproj]

This is related to testhost not being an .exe on non-Windows. We should fix that and then protect that path on CI to not regress again.

Ideally this repository would stop using nuspecs and just use the normal NuGet Pack task which would make things much easier to condition on for different build modes (i.e. source-build vs VMR vs repo build).