`dotnetUp` hostfxr / muxer replacement logic is incomplete

[nagilson]

The current logic which replaces the muxer looks at existing sdk versions, which is what dnup does. However, its theoretically possible the user is in a bad state where they copy a hostfxr into the folder that's not a version which matches the existing SDKs. If you downgrade the SDK, the MSI install will downgrade the version of the hostfxr (which is all good.)

Ideally, we can look at the assembly version of the hostfxr / dotnet.exe itself to determine this instead.

We also need to consider the license.md file and the TPN (third party notice) when we do the extraction and consider which version of those files to use.