overrideAttrs has no effect?

Perhaps a dumb question but I’m running into some problems overriding a package. For the following two files:

neovim.nix

# { neovim }:
{ pkgs ? import <nixpkgs> {} }:

pkgs.neovim-unwrapped.overrideAttrs (oldAttrs: {
  NIX_CFLAGS_COMPILE = "test"; #" -O3 -march=native";
  buildPhase = ''
    echo "======================================"
    echo $NIX_CFLAGS_COMPILE
    echo "======================================"
  '';
  enableParallelBuilding = true;
})

builder.nix

{ pkgs ? import <nixpkgs> {}}:
pkgs.callPackage ./neovim.nix {}

My overrides have no effect when calling nix-build --no-out-link builder.nix while if I clone nixpkgs and edit “pkgs/applications/editors/neovim/default.nix” directly I am able to (obviously) modify the derivtion. Any ideas?

EDIT: So I was partially mistaken, it appears overriding buildPhase and NIX_CFLAGS_COMPILE DOES have an effect, but not enableParallelBuilding.

What’s even weird is:

  1. Clone nixpkgs
  2. nix-build builder.nix --no-out-link (Doesn’t build parallel by default)
  3. add “enableParallelBuilding” to pkgs/applications/editors/neovim/default.nix`
  4. nix-build builder.nix --no-out-link (NOW it builds in parallel, but only if edited directly not via overrideAttrs)
  5. REMOVE “enableParallelBuilding” from pkgs/applications/editors/neovim/default.nix`
  6. nix-build builder.nix --no-out-link (STILL BUILDS IN PARALLEL)

What the heck?