Fix dotnet path for RoslynCodeTaskFactory

[GangWang01]

Fixes #9052

Context

For RoslynCodeTaskFactory, dotnet path is wrongly set using the path of the application when running the application via application.exe.

Changes Made

Get dotnet path through the following in order.

1. Honor the environment variable DOTNET_HOST_PATH for tools and MSBuild tasks within the SDK to ensure a consistent experience.
2. Fallback to get dotnet path from current process which might be dotnet executable.
3. If the path resolved above is not dotnet executable, rely on dotnet via the system's PATH.

Testing

Add a test that verifies build the task of RoslynCodeTaskFactory using API.

Notes

[rainersigwald]

Just a couple of nits, but looks good!

[GangWang01]

/azp run

[azure-pipelines]

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