vim: do not `expand()` shell vars

This is especially important when testing for variables like `$TMP`: if
they do not exist, a new shell is started to look for them, although
shell variables are provided already in Vim.

I might be missing something here, e.g. regarding Windows support!

See also: http://www.reddit.com/r/vim/comments/2el2zo/til_expandshellvar_might_spawn_a_shell/