squashfs: use -no-hardlinks for reproducible squashfs images (#114454)

the nix store may contain hardlinks: derivations may output them
directly, or users may be using store optimization which automatically
hardlinks identical files in the nix store.

The presence of these links are intended to be a 'transparent'
optimization. However, when creating a squashfs image, the image
will be different depending on whether hard links were present
on the filesystem, leading to reproducibility problems.

By passing '-no-hardlinks' to mksquashfs the files are stored
as duplicates in the squashfs image. Since squashfs has support
for duplicate files this does not lead to a larger image.

For more details see
https://github.com/NixOS/nixpkgs/issues/114331

nixos/lib/make-squashfs.nix

@@ -23,6 +23,6 @@ stdenv.mkDerivation {
 
       # Generate the squashfs image.
       mksquashfs nix-path-registration $(cat $closureInfo/store-paths) $out \
-        -keep-as-directory -all-root -b 1048576 -comp ${comp}
+        -no-hardlinks -keep-as-directory -all-root -b 1048576 -comp ${comp}
     '';
 }