dotnetup: Re-enable installation validation via HostFxr on Unix platforms

[dsplaisted]

We're having issues correctly calling into HostFxr from dotnetup on Linux. So for now I am going to disable the hostfxr-based installation validation except on Windows. Once we get the pinvokes working correctly we should re-enable this.

Related: #51689

[nagilson]

Do you have the error message on hand? Since this is linked to this code: #52821 I assume it's different from the extraction error you saw here: #51689 (comment) since that's an issue with the muxer being in use (e.g. the sdk is running and using that dotnet muxer to run under so we couldn't replace it, meanwhile this code suggests an issue calling into the muxer itself to verify installs after its been perhaps replaced)