make-squashfs: Only limit cores to 48 on i686

There shouldn't be a need to limit it on other systems

nixos/lib/make-squashfs.nix

@@ -1,4 +1,4 @@
-{ stdenv, squashfsTools, closureInfo
+{ lib, stdenv, squashfsTools, closureInfo
 
 , # The root directory of the squashfs filesystem is filled with the
   # closures of the Nix store paths listed here.
@@ -22,11 +22,13 @@ stdenv.mkDerivation {
       # for nix-store --load-db.
       cp $closureInfo/registration nix-path-registration
 
+    '' + lib.optionalString stdenv.buildPlatform.is32bit ''
       # 64 cores on i686 does not work
       # fails with FATAL ERROR: mangle2:: xz compress failed with error code 5
       if ((NIX_BUILD_CORES > 48)); then
         NIX_BUILD_CORES=48
       fi
+    '' + ''
 
       # Generate the squashfs image.
       mksquashfs nix-path-registration $(cat $closureInfo/store-paths) $out \