Add a flag to `msys2_shell.cmd` for disabling login

[mon-jai]

Describe the issue

As per #2882, it takes about 3-4 seconds to stratup msys2 bash with msys2_shell.cmd.

"C:/msys64/msys2_shell.cmd" -defterm -here -no-start -msys

But non-login bash starts up almost immediately.

cmd /k set MSYSTEM=MSYS&& set PATH=C:/msys64/usr/bin;%PATH%&& bash

Allowing disabling login will speed up msys2 launch for some cases where login isn't necessary.