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

pkgs/tools/filesystems/squashfs/default.nix

@@ -19,6 +19,10 @@ stdenv.mkDerivation {
     # This patch adds an option to pad filesystems (increasing size) in
     # exchange for better chunking / binary diff calculation.
     ./4k-align.patch
+    # Add -no-hardlinks option. This is a rebased version of
+    # c37bb4da4a5fa8c1cf114237ba364692dd522262, can be removed
+    # when upgrading to the next version after 4.4
+    ./0001-Mksquashfs-add-no-hardlinks-option.patch
   ] ++ lib.optional stdenv.isDarwin ./darwin.patch;
 
   buildInputs = [ zlib xz zstd lz4 ];