Zsh looks for .zshrc in $ZDOTDIR, or $HOME if $ZDOTDIR is unset. I doubt
non-$HOME $ZDOTDIRs are widespread (I might be the only one I know with
one), but it's how zsh's startup is defined, so it can happen.
This commit simply adds a check to the */zsh case of find_shell() to
respect $ZDOTDIR if it is set.
