With this function, I can use the same logic in other configuration
files (e.g. `~/.local/etc/zsh/config.zsh`). This avoids duplication in
tmux sessions when prepending naively.
`~/.zshenv' is executed for _every_ instance of `zsh', so we should
limit the work done there to the absolute minimum. For me, that
currently means only making sure `$ZDOTDIR' is set so that login (via
`.zprofile') and interactive (via `.zshrc') shells can be configured
appropriately.
