stdenv/darwin: allow brotli reference from curl

[thefloweringash]

Brotli is recently a default dependency of curl in nixpkgs.

See e3d1967 in #112947

Motivation for this change

Fix darwin stdenv build.

Stage 2 now references brotli:

$ nix why-depends --all /nix/store/4x2440ns4gzyknaa70lzrk832vsn3djv-bootstrap-stage2-stdenv-darwin   /nix/store/qvv2xw5da22lfgbj1zq4hhvh286i74f7-brotli-1.0.9-lib
/nix/store/4x2440ns4gzyknaa70lzrk832vsn3djv-bootstrap-stage2-stdenv-darwin
╚═══setup: ….defaultBuildInputs="/nix/store/irylbhn1qz80fyqlcjcxl0jaqm2yr328-swift-corefoundation".# Don't p…
    => /nix/store/irylbhn1qz80fyqlcjcxl0jaqm2yr328-swift-corefoundation
    ╚═══Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation: ….h.................../nix/store/1ycixpvd73ys8lzpkf2562pdxz5dxfp6-curl-7.74.0/lib/libcurl.4.dylib…
        => /nix/store/1ycixpvd73ys8lzpkf2562pdxz5dxfp6-curl-7.74.0
        ╚═══lib/libcurl.4.dylib: ….p.................../nix/store/qvv2xw5da22lfgbj1zq4hhvh286i74f7-brotli-1.0.9-lib/lib/libbrotlid…
            => /nix/store/qvv2xw5da22lfgbj1zq4hhvh286i74f7-brotli-1.0.9-lib

Comparing master and this branch (shameless plug for my stdenv analyser), the new builds by stage are:

• libidn-1.36.drv at stages 1 and 4
• brotli-1.0.9.drv at stages 1 and 4

And libidn doesn't appear in the closure of curl, so can be omitted here.

This isn't the only solution, we could use curlMinimal in stage1 .

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.