Backport Zsh fpath child shell and nounset fixes from NixOS

[antoineco]

Follow up to #1066

Backports the following two commits from NixOS:

• NixOS/nixpkgs@55819e6c861f53450030eea832a76583a6786370: allows children Zsh shells to set their fpath.

  Without this fix, a child shell opened with a command such as zsh -il or nix develop -c zsh only contains Zsh's default fpath entry, and none of Nix's profiles fpath entries.

  • Before, opening a child shell causes Nix's fpath entries to get skipped:

    $ echo $fpath
    /opt/homebrew/share/zsh/site-functions /Users/acotten/.nix-profile/share/zsh/site-functions /Users/acotten/.nix-profile/share/zsh/5.9/functions /Users/acotten/.nix-profile/share/zsh/vendor-completions /etc/profiles/per-user/acotten/share/zsh/site-functions /etc/profiles/per-user/acotten/share/zsh/5.9/functions /etc/profiles/per-user/acotten/share/zsh/vendor-completions /run/current-system/sw/share/zsh/site-functions /run/current-system/sw/share/zsh/5.9/functions /run/current-system/sw/share/zsh/vendor-completions /nix/var/nix/profiles/default/share/zsh/site-functions /nix/var/nix/profiles/default/share/zsh/5.9/functions /nix/var/nix/profiles/default/share/zsh/vendor-completions /nix/store/fq0zsssivzwkm4cj05wp8imm386l9gx8-zsh-5.9/share/zsh/5.9/functions
    $ zsh -il
    $ echo $fpath
    /opt/homebrew/share/zsh/site-functions /nix/store/fq0zsssivzwkm4cj05wp8imm386l9gx8-zsh-5.9/share/zsh/5.9/functions

  • After, fpath is properly re-populated with Nix's entries:

    $ echo $fpath
    /opt/homebrew/share/zsh/site-functions /Users/acotten/.nix-profile/share/zsh/site-functions /Users/acotten/.nix-profile/share/zsh/5.9/functions /Users/acotten/.nix-profile/share/zsh/vendor-completions /etc/profiles/per-user/acotten/share/zsh/site-functions /etc/profiles/per-user/acotten/share/zsh/5.9/functions /etc/profiles/per-user/acotten/share/zsh/vendor-completions /run/current-system/sw/share/zsh/site-functions /run/current-system/sw/share/zsh/5.9/functions /run/current-system/sw/share/zsh/vendor-completions /nix/var/nix/profiles/default/share/zsh/site-functions /nix/var/nix/profiles/default/share/zsh/5.9/functions /nix/var/nix/profiles/default/share/zsh/vendor-completions /nix/store/fq0zsssivzwkm4cj05wp8imm386l9gx8-zsh-5.9/share/zsh/5.9/functions
    $ zsh -il
    $ echo $fpath
    /opt/homebrew/share/zsh/site-functions /Users/acotten/.nix-profile/share/zsh/site-functions /Users/acotten/.nix-profile/share/zsh/5.9/functions /Users/acotten/.nix-profile/share/zsh/vendor-completions /etc/profiles/per-user/acotten/share/zsh/site-functions /etc/profiles/per-user/acotten/share/zsh/5.9/functions /etc/profiles/per-user/acotten/share/zsh/vendor-completions /run/current-system/sw/share/zsh/site-functions /run/current-system/sw/share/zsh/5.9/functions /run/current-system/sw/share/zsh/vendor-completions /nix/var/nix/profiles/default/share/zsh/site-functions /nix/var/nix/profiles/default/share/zsh/5.9/functions /nix/var/nix/profiles/default/share/zsh/vendor-completions /nix/store/fq0zsssivzwkm4cj05wp8imm386l9gx8-zsh-5.9/share/zsh/5.9/functions

• NixOS/nixpkgs@7d84dbdf5b91439f798363559310d86b21bfa86c: prevents noisy failures upon loading /etc/zshenv and /etc/zprofile when the shell has the nounset option set.

  /etc/zshenv:6: __ETC_ZSHENV_SOURCED: parameter not set
  /etc/zshrc:5: __ETC_ZSHRC_SOURCED: parameter not set