make-squashfs: use $NIX_BUILD_CORES or 48 cores if above 48
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/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs