Fixes #429

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.

See https://github.com/commercialhaskell/stack/issues/5888

Fixes #407.

Added and referenced solution in case of 'ghc version not found' for Linux VSCode + hls extension users.