Check $ZDOTDIR in bootstrap script

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.