Don't recommend setting PATH in .zshenv

[johnpryan]

The official instructions recommend using .zshenv to set the PATH variable, but this doesn't work on macOS because something called path_helper runs after .zshenv and overwrites the PATH variable:

More details here: https://gist.github.com/Linerre/f11ad4a6a934dcf01ee8415c9457e7b2

Docs page: https://docs.flutter.dev/install/add-to-path#macos

We should recommend setting the PATH variable in .zprofile instead (for macOS).