squashfsTools: apply reproducibility patches

Without these patches, the output of mksquashfs is not reproducible.
The patches are taken from https://github.com/squashfskit/squashfskit,
a fork of squashfs-tools, licensed under the GPL2 or later like
squashfs-tools itself.

pkgs/tools/filesystems/squashfs/default.nix

@@ -15,6 +15,15 @@ stdenv.mkDerivation rec {
     rev = "9c1db6d13a51a2e009f0027ef336ce03624eac0d";
   };
 
+  # These patches ensures that mksquashfs output is reproducible.
+  # See also https://reproducible-builds.org/docs/system-images/
+  # and https://github.com/NixOS/nixpkgs/issues/40144.
+  patches = [
+    ./0001-If-SOURCE_DATE_EPOCH-is-set-override-timestamps-with.patch
+    ./0002-If-SOURCE_DATE_EPOCH-is-set-also-clamp-content-times.patch
+    ./0003-remove-frag-deflator-thread.patch
+  ];
+
   buildInputs = [ zlib xz ]
     ++ stdenv.lib.optional lz4Support lz4;