make-squashfs: use $NIX_BUILD_CORES or 48 cores if above 48

by default all cores are used

hoping this will fix the hydra i686 squashfs build issues as all the
failures were using 64 cores

Parallel mksquashfs: Using 64 processors
Creating 4.0 filesystem on ..., block size 1048576.
FATAL ERROR: mangle2:: xz compress failed with error code 5

nixos/lib/make-squashfs.nix

@@ -21,8 +21,15 @@ stdenv.mkDerivation {
       # for nix-store --load-db.
       cp $closureInfo/registration nix-path-registration
 
+      # 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 \
-        -no-hardlinks -keep-as-directory -all-root -b 1048576 -comp ${comp}
+        -no-hardlinks -keep-as-directory -all-root -b 1048576 -comp ${comp} \
+        -processors $NIX_BUILD_CORES
     '';
 }