Pin default shell to cmd.exe on windows

[ysl2]

I have set my default shell to powershell 7 (let &shell='pwsh'), but the command in function s:SaveFileTMPWin32 is cmd.exe format.
So I realized we should backup the original shell first (defined by &shell and &shellcmdflag, run :h system for more information), then run the "powershell paste image" command, finally we should recover the original shell. If we don't change the &shell to cmd.exe, it will return 1 indicates the command failed.