The following 'vim-plug' commands would fail and/or display errors
when executed on by VIM 8+ on a Windows platform with the 'shell'
option set to 'powershell' or 'pwsh':
  - PlugInstall
  - PlugUpdate
  - PlugClean
  - PlugStatus
  - PlugDiff

There were two causes for these errors:
  - A bug in VIM itself (resolved by patch 9.2.6) with how compound
    PowerShell commands are handled by the 'system()' vimscript
    function.
  - A bug in the 's:vim8' branch of the private 'vim-plug' function
    's:spawn()' where the actual command to be executed was escaped
    for 'cmd.exe' but the 'cd' prefix added by 's:with_cd()" was escaped
    for PowerShell.