DotNetExe FullPathOrDefaultis returning invalid path

[iskiselev]

Describe the bug

DotNetExe.FullPathOrDefault() should return full path or just dotnet. But TryFindDotNetExePath() always return some string (with fallback to "/usr/local/share/dotnet/dotnet" - which is invalid at least for Alpine).

Should it check if file exists anr return null if not - to allow FullPathOrDefault return just dotnet?