Installing developer preview didn't set up PATH correctly on zsh

REPOST FROM: https://github.com/ocaml/dune/issues/10963 by @edwintorok

Expected Behavior

$ dune --version
"Dune Developer Preview: build 2024-09-28T01:30:13+00:00, git revision
17071ec30d10390badcb6cb1f6a43984b1be54a6"
$ echo $PATH
/var/home/edwin/.dune/bin:/var/home/edwin/.opam/5.2.0+fp/bin:/var/home/edwin/.cargo/bin:/var/home/edwin/.local/bin:/var/home/edwin/.dune/bin:/var/home/edwin/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:/var/home/edwin/.dotnet/tools

Actual Behavior

$ dune --version
3.16.0
$ zsh -l
$ echo $PATH
/var/home/edwin/.opam/5.2.0+fp/bin:/var/home/edwin/.cargo/bin:/var/home/edwin/.local/bin:/var/home/edwin/.nix-profile/bin:/var/home/edwin/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:/var/home/edwin/.dotnet/tools
$ zsh
$ echo $PATH
/var/home/edwin/.opam/5.2.0+fp/bin:/var/home/edwin/.cargo/bin:/var/home/edwin/.local/bin:/var/home/edwin/.nix-profile/bin:/var/home/edwin/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:/var/home/edwin/.dotnet/tools

Reproduction

You can find my full ZSH config here:
https://gitlab.com/edwintorok/dotfiles/-/tree/master/zsh?ref_type=heads

However for the purposes of this bug you can repro it with:

• PR with a reproducing test:

1. opam install dune
2. echo 'return 0' >.zshrc
3. curl https://dune.ci.dev/install | bash
   1.zsh -l
4. dune --version
5. echo $PATH

Specifications

• Version of dune (output of dune --version): 3.16.0
• Version of ocaml (output of ocamlc --version): 5.2.0
• Operating system (distribution and version): Fedora 40

Additional information

Dune added this to my .zshrc, but that has no effect

# dune
export PATH="/var/home/edwin/.dune/bin:$PATH"

If I instead I add it to .zshenv it is better, but still in the wrong place (opam takes precedence):

/var/home/edwin/.opam/5.2.0+fp/bin:/var/home/edwin/.dune/bin:/var/home/edwin/.cargo/bin:/var/home/edwin/.local/bin:/var/home/edwin/.nix-profile/bin:/var/home/edwin/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:/var/home/edwin/.dotnet/tools

The reason it doesn't work in .zshrc because mine ends like this:

# Setup highlighting {{{
# Must be last according to its docs
ZSH_HIGHLIGHT_HIGHLIGHTERS=(main brackets)
#SYNTAX=/usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
SYNTAX=~/dotfiles/zsh/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
[[ -s ${SYNTAX} ]] && . ${SYNTAX}
return 0

So I need to move the export PATH before the 'return 0', so this works:

export PATH="/var/home/edwin/.dune/bin:$PATH"

# Setup highlighting {{{
# Must be last according to its docs
ZSH_HIGHLIGHT_HIGHLIGHTERS=(main brackets)
#SYNTAX=/usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
SYNTAX=~/dotfiles/zsh/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
[[ -s ${SYNTAX} ]] && . ${SYNTAX}
return 0

Start by inspecting the installer fetched from https://dune.ci.dev/install and reproduce the setup with a minimal .zshrc containing return 0. Check how the installer chooses and updates zsh startup files; done means a new login and interactive zsh session resolve the installed developer-preview dune before the opam version.