Don't re-append the subdir to the store path

[grahamc]

In #81, we inadvertently appended the subdir to the path which already included the subdir when reading the readme. This causes publishing failures because the directory we expect the readme to exist in likely does not exist:

 INFO flakehub_push:push_new_release: Preparing release of grahamc/nix-starter-flakes/0.0.0 repository=grahamc/nix-starter-flakes mirror=false upload_name=grahamc/nix-starter-flakes
flake_store_path: "/nix/store/abg459275ipf6x32an2padhjjclx26ip-source/go"
readme_path: "/nix/store/abg459275ipf6x32an2padhjjclx26ip-source/go/go"

[cole-h]

Hm, I could have sworn there was a reason I kept that, but indeed it's broken on my flake-inside-a-directory test case... Oops.