Use nethost.dll to find dotnet in ANCM

[jkotalik]

Fixes #8838

[natemcmaster]

Changes LGTM. If it works,

[jkotalik]

Got a slight block on dotnet/core-setup#6672

[jkotalik]

@BrennanConroy this is now ready for review. No rush though, I know SignalR + Npm is higher priority.

[jkotalik]

Re-adding a blocked label to this. I believe there is a fundamental issue with nethost.dll where it can't accept a path to dotnet. This will break anyone who doesn't specify DOTNET_ROOT, DOTNET_HOME, or dotnet on the path and pass in an absolute path to dotnet.

[jkotalik]

This will be mostly unblocked by dotnet/core-setup#7020 once we update dependencies.

We will need to keep our logic to find where dotnet is if it is only available via the path. We can still feed the dotnet location to nethost.dll to find hostfxr, but nethost will not path probe.

Here is some pseudo code for how we would find dotnet now:

• Call nethost passing in either the path to the application dll and/or the path to dotnet.exe.
• If dotnet.exe isn't found, try to find dotnet on PATH.
• If we find dotnet.exe, call nethost.dll again with the newly found path to dotnet.

[dougbu]

@jkotalik will core-setup packages with version 3.0.0-preview8-27904-06 get you what you need? That was the latest when I manually updated the dependencies a couple of days ago (see #11939).

[jkotalik]

Closing for now, will reopen one day....