nixos/zsh: Adds enableGlobalCompInit option

[Gerschtli]

Motivation for this change

My current zsh setup requires setting the fpath variable in my local ~/.zshrc. Therefore I need to run compinit after the fpath updates. I can't disable the compinit call without removing other zsh completion files, so I introduced enableGlobalCompInit for this exact purpose.

I need to disable the compinit call in /etc/zshrc because on slow devices the startup time of my shell can be remarkably decreased by calling compinit only in my local config

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

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[Mic92]

Can you add, that this option can be used, if the user wants to extend its fpath.

[Gerschtli]

Done