iso-image: enable XZ compress filter

Minimal ISO:
1m21 -> 2m25
625M -> 617M

Plasma5 ISO:
2m45 -> 5m18
1.4G -> 1.3G

Decompression speed stays about the same. It's just a few seconds for the whole
image anyways and, with that kind of speed, you're going to be bottlenecked by
IO long before the CPU.

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -425,7 +425,12 @@ in
     };
 
     isoImage.squashfsCompression = mkOption {
-      default = "xz -Xdict-size 100%";
+      default = with pkgs.stdenv.targetPlatform; "xz -Xdict-size 100% "
+                + lib.optionalString (isx86_32 || isx86_64) "-Xbcj x86"
+                # Untested but should also reduce size for these platforms
+                + lib.optionalString (isAarch32 || isAarch64) "-Xbcj arm"
+                + lib.optionalString (isPowerPC) "-Xbcj powerpc"
+                + lib.optionalString (isSparc) "-Xbcj sparc";
       description = ''
         Compression settings to use for the squashfs nix store.
       '';