fix for failing substition & eliminated unnecessary cd

Fixes https://github.com/robbyrussell/oh-my-zsh/issues/1705#issuecomment-16887176
and https://github.com/robbyrussell/oh-my-zsh/issues/1744 (kind of)

Changing the working directory in a sub-subshell
does not change the working directory of the executing
shell.

The substitution was broken for me on _all_ my machines,
so I started looking into the business.