Add override

modules/linuxcnc.nix

@@ -2,9 +2,12 @@
 let
   cfg = config.local.packages.linuxcnc;
   inherit (builtins) filter map pathExists listToAttrs;
+  linuxcncWithEC = pkgs.linuxcnc.overrideAttrs (oldAttrs: {
+    buildInputs = oldAttrs.buildInputs ++ [ pkgs.linuxcnc-ethercat pkgs.hal-cia402 ];
+  });
   linuxcncEnv = pkgs.buildEnv {
     name = "linuxcnc-env";
-    paths = with pkgs; [ linuxcnc hal-cia402 linuxcnc-ethercat ];
+    paths = with pkgs; [ linuxcncWithEC hal-cia402 linuxcnc-ethercat ];
   };
 in {
   options.local.packages.linuxcnc.enable = lib.mkEnableOption "Enable linuxcnc";

packages/linuxcnc-ethercat/default.nix

@@ -14,17 +14,12 @@ stdenv.mkDerivation rec {
   };
 
   nativeBuildInputs = with pkgs; [
-    autoPatchelfHook
     which
     expat
   ];
 
   buildInputs = [ linuxcnc ethercat ];
 
-  preBuild = ''
-    #addAutoPatchelfSearchPath ${placeholder "out"}/lib/linuxcnc/modules/
-  '';
-
   installFlags = [
     "prefix=${placeholder "out"}"
     #"RUN_IN_PLACE=YES"