dockerTools: Properly add /nix/ and /nix/store/ first to layer.tar

In https://github.com/NixOS/nixpkgs/pull/58431 the authors ensured that
the resulting layer.tar would always list

  /nix/
  /nix/store/

first to fully comply to the tar spec. Various refactorings later it is only
ensured to create /nix/ but NOT /nix/store anymore. Instead tar transformed
them to /nix/nix and /nix/nix/store.

pkgs/build-support/docker/store-path-to-layer.sh

@@ -30,12 +30,13 @@ mkdir -p nix/store
 # store path to the absolute store path.
 tarhash=$(
   basename -a "$@" |
-    tar -cp nix \
-      -C /nix/store --verbatim-files-from --files-from - \
+    tar --create --preserve-permissions --absolute-names nix \
+      --directory /nix/store --verbatim-files-from --files-from - \
       --hard-dereference --sort=name \
       --mtime="@$SOURCE_DATE_EPOCH" \
       --owner=0 --group=0 \
-      --transform 's,^nix(/|$),/nix/,' \
+      --transform 's,^nix$,/\0,' \
+      --transform 's,^nix/store$,/\0,' \
       --transform 's,^[^/],/nix/store/\0,rS' |
     tee "$layerPath/layer.tar" |
     tarsum