These commands are currently broken on powershell (Windows) due to
improper use of the InvokeCommand commandlet and a lack of direct
support for the `--pwsh` argument in `spack load`, `spack unload`,
and `spack env deactivate`.
PowerShell requires explicit shell and env support in Spack.
This is due to the distinct differences in shell interactions between
cmd and pwsh. Add a doskey in pwsh piping 'spack' commands to a
powershell script similar to the sh function 'spack'. Add
support for PowerShell-specific shell interactions from Spack
(set/unset shell variables).
