VSIX is only generated using .NET Framework MSBuild, not Core

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

Running `dotnet build` doesn't produce a VSIX, only the full framework `msbuild.exe` does. That's because Core MSBuild is disabled here:

https://github.com/dotnet/arcade/blob/00bb044268c007ed27c5ab50c5839fcf36fdff0f/src/Microsoft.DotNet.Arcade.Sdk/tools/Imports.targets#L18

I'm not sure what's the reason for that because having something like the following in the project file makes `dotnet build` produce vsix as well:

```csproj

42.42.42.4242424
false

$(VersionPrefix).$(VersionSuffixDateStamp)$(VersionSuffixBuildOfTheDayPadded)

```