Set dotnet_root_<arch> always

[nohwnd]

Description

Always set DOTNET_ROOT_ (e.g. DOTNET_ROOT_X64) when running under dotnet test, to make sure that the location of dotnet".exe", propagates to the testhost and child processes when we are on windows, linux and macos. This allows local installations of dotnet to be used to run tests that run child executables. On Windows this is alread in place for a long time when testhost.exe is found, so it can resolve correctly dotnet that is not in program files.

On Linux and MacOS it will be used to run xunit v3 tests.

There is a feature flag to disable this new behavior: VSTEST_DISABLE_DOTNET_ROOT_ON_NONWINDOWS=1

Related issue

Will fix dotnet/sdk#50331 once merged and flown.

Working:

Reverting to broken behavior with the feature flag:

[nohwnd]

note for me:

tested on wsl with rc1 release , patched with the newly built testruntime provider.
cp /mnt/s/p/vstest/artifacts/bin/Microsoft.TestPlatform.TestHostProvider/Debug/netstandard2.0/Microsoft.TestPlatform.TestHostRuntimeProvider.dll dotnet3/sdk/10.0.100-rc.1.25418.110/Extensions/Microsoft.TestPlatform.TestHostRuntimeProvider.dll

[Youssef1313]

Diff is kinda hard to follow with, but reading the overall logic, LGTM.