Calling powershell.exe in PSCore6 on Windows starts Windows PowerShell

On Windows, $env:Path has Windows PowerShell in the path before $PSHOME which means that if you’re in PowerShell Core 6 and type powershell, it starts Windows PowerShell 5.1 instead of a nested instance of PowerShell Core 6. This may be unexpected for end users as it seems that one would expect powershell to refer to the currently running version of PowerShell.

The immediate impact is that partner teams running existing PowerShell script tests in PowerShell Core 6 that spawn powershell will get incorrect results as that process is running Windows PowerShell.

cc @joeyaiello @HemantMahawar

Steps to reproduce

# within PowerShell Core 6
powershell
$psversiontable

Expected behavior

PowerShell Core 6

Actual behavior

Windows PowerShell 5.1

Environment data

Name                           Value
----                           -----
PSVersion                      6.0.0-beta
PSEdition                      Core
GitCommitId                    v6.0.0-beta.3
OS                             Microsoft Windows 10.0.16225
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

[iSazonov]

@SteveL-MSFT Maybe Area-SideBySide?

[SteveL-MSFT]

If we decide to do https://github.com/PowerShell/PowerShell/issues/4214, we don’t need to address this one