Add parameter for .NET installation/binaries directory path

[MichalMucek]

Add a parameter to NUnit Console for .NET installation/binaries directory path that, when defined, will omit or extend NUnit.Engine.Internal.TestAssemblyResolver.GetDotNetInstallDirectory().

The motivation behind this feature is to enable the NUnit Console to work in Windows environments where .NET is not installed but only .NET binaries are downloaded, e.g. during pipeline execution.

[stevenaw]

@MichalMucek Are the binaries used in your pipeline for net framework or core/net5+?

For the latter, I think the DOTNET_ROOT environment variable would be the standard way to define this if the console runner were updated to support this (https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-environment-variables#dotnet_root-dotnet_rootx86).