New spawned shell's PATH is random ordered on Windows

[davidlatwe]

Problem

New spawned cmd and powershell has random ordered PATH injected, the order of paths in PATH should be respected or the behavior in the new shell might be unexpected.

Cause

These two lines here use set to cleanup duplicated path, the motivation was good but the ordering is undesired.
https://github.com/nerdvegas/rez/blob/def201aca819862470b879d5beb75fcf3d46f87f/src/rezplugins/shell/cmd.py#L135
https://github.com/nerdvegas/rez/blob/def201aca819862470b879d5beb75fcf3d46f87f/src/rezplugins/shell/_utils/powershell_base.py#L123

Since my machine at home was pretty nasty plus WindowsApps' fake python.exe (see tweet), this random ordered PATH was pretty obvious to me.