PR Summary

This PR is a re-implementation of the -AddToPath switch in tools/install-powershell.ps1, without the side-effects the current implementation has. The changes only affects windows users.

Summarized changes

As currently implemented, the -AddToPath switch has quite a few undesirable side-effects and 1 or 2 issues that prevents it from adding the path to the newly installed powershell to the registry. Specifically:

• it expands all variables in the path to their full value (%SystemRoot% ==> C:\Windows for example) and saves the new path with all variables expanded.
• it changes the path type from REG_EXPAND_SZ to REG_SZ, disabling all variables (wich are now gone in any event).
• it fails o add the path to the newly installed powershell if the user has a path that has the same base path (it won't add c:\pwsh of you have c:\pwsh6 in your path).
• it will add a duplicate path for path values that have a DirectorySeparatorChar at the end (for example, it will add c:\pwsh6\ in your path even if you have c:\pwsh6).

This PR remedies all of the above by

• strictly adding the new path to the current path value, without changing the current path value in any other way.
• saving the new path with the same REG_EXPAND_SZ/REG_SZ it had before, if appropriate.
• not adding a duplicate path by understanding that C:\1 and C:\1\ is the same path.

Non-Breaking change

• The updated script will attempt to add the path to the newly installed pwsh in the user's path. This is entirely new, as the previous version only attempted to save the path in the machine wide path, and very likely failed most of the time, lacking sufficient rights to do so.
• This last bit also explains why these issues and side-efects have gone unnoticed until now ... you have to be in an elevated console for the path-rewrite to happen. Since the script is mostly used while building (via build.psm1), and since most people don't use an elevated console for building, ... I beleive few users were affected.

An alternate fix (dont do anything for -AddToPath when $WinEnv -eq $true)

• I am not convinced of the pertinence of saving the path to the registry. It isn't required by the build script (adding the path to the current process is enough to satisfy it's needs). It depends who the target audience for install-powershell.ps1 is. If the script is used by the public at large as an easy way to install powershell, then it's pertinent. If it's really only used by the build script, then I wasted quite a bit of time getting this right, and the -AddToPath switch should not do anything for $WinEnv -eq $true.