kicad.libraries.packages3d: use stepreduce & stepZ

this makes it buildable by hydra

stepreduce brings down closure size from 5.4G to 3.3G
zip brings it down from 3.3G to 1.6G (or 5.4G to 1.9G without reduce)

xz compressed NAR sizes are:
plain: 373M
gzip: 712M
zip: 712M
stepreduce: 262M
stepreduce & gzip: 465M
stepreduce & zip: 468M

pkgs/applications/science/electronics/kicad/libraries.nix

@@ -2,6 +2,9 @@
 , cmake
 , gettext
 , libSrc
+, stepreduce
+, parallel
+, zip
 }:
 let
   mkLib = name:
@@ -11,14 +14,20 @@ let
 
       src = libSrc name;
 
-      nativeBuildInputs = [ cmake ];
+      nativeBuildInputs = [ cmake ]
+        ++ lib.optionals (name == "packages3d") [
+          stepreduce
+          parallel
+          zip
+        ];
+
+      postInstall = lib.optional (name == "packages3d") ''
+        find $out -type f -name '*.step' | parallel 'stepreduce {} {} && zip -9 {.}.stpZ {} && rm {}'
+      '';
 
       meta = rec {
         license = lib.licenses.cc-by-sa-40;
         platforms = lib.platforms.all;
-        # the 3d models are a ~1 GiB download and occupy ~5 GiB in store.
-        # this would exceed the hydra output limit
-        hydraPlatforms = if (name == "packages3d") then [ ] else platforms;
       };
     };
 in