Adding an overlay for supercollider with plugins

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So we have a package called supercollider; and that is helpfully available to us as a package.
Now; I’ve written something to build sc3plugins; and this is my nix derivation; which seems to build without issues.

{ pkgs ? (import <nixpkgs> { }) }:
let name = "sc3-plugins";
in pkgs.stdenv.mkDerivation rec {

  inherit name;

  # https://github.com/supercollider/sc3-plugins/commit/7d08fab319198f27a5368307d0e9833c41b8053b
  # date = "2020-06-05T09=30=19-07:00";
  src = pkgs.fetchgit {
    url = "git@github.com:supercollider/sc3-plugins.git";
    rev = "7d08fab319198f27a5368307d0e9833c41b8053b";
    sha256 = "0qxn1dx9ccb5bkxx0mj2nxh5iqvk3xcx23rclqbc3kxv6gn41rwl";
    fetchSubmodules = true;
  };

  buildInputs = with pkgs; [ cmake supercollider fftw libsndfile ];

  cmakeFlags = with pkgs; [
    "-DSUPERNOVA=ON"
    "-DSC_PATH=${supercollider}/include/SuperCollider"
    "-DFFTW3F_LIBRARY=${fftw}/lib/"
  ];
  
  buildPhase = ''
    cmake .
    cmake --build . --config Release
    cmake --build . --config Release --target install
  '';

}

Now; to be able to use the plugins in super collider; these plugins need to be kept in
${pkgs.supercollider}/share/SuperCollider/Extensions.

I’m now trying to write an overlay that can achieve that; but I feel totally lost on how to achieve it.

My line of thought so far is to have this in the installPhase of the supercollider derivation; something on the lines of:

self: super: 
let 
  # the plugins must be added to share/SuperCollider/Extensions 
  plugins = (import ./sc3plugins.nix) { pkgs = super; } ; 
  
  overrideOld = oldAttrs: oldAttrs // { 
    installPhase = ''
    ls ${super.supercollider}
    ls $plugins
    '';
  };

in { 
  
  supercollider = super.supercollider.overrideAttrs ( overrideOld );

}

When trying to run the above in a REPL; things fail:

nix-repl> pkgs = import <nixpkgs> { overlays = [ (import ./supercollider.nix) ]; } 
nix-repl> :b pkgs.supercollider
builder for '/nix/store/xn073lkx5ky75zmy6nmqqdxswcmghnxq-supercollider-3.10.4.drv' failed to produce output path '/nix/store/qy5h87c1hlv6piq96lnqjnmcd89kq6gv-supercollider-3.10.4'
[0 built (1 failed), 4 copied (15.2 MiB), 14.8 MiB DL]
error: build of '/nix/store/xn073lkx5ky75zmy6nmqqdxswcmghnxq-supercollider-3.10.4.drv' failed

Now; given that my installPhase looks relatively harmless; I find it confusing as to why this fails.

And regarding this; I have several questions.

  1. How does one test overlays?
    For example; I can do nix-build foobar.nix --keep-failed to peek around for normal derivations. But that assumption doesn’t work inside a repl.
  2. Can someone nudge me in the right direction here? How can I say; in an overlay; that whatever the build output of a given package is; also add the following things to some paths under it; or is this considered bad practice?
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