v5.1.0

• Use a custom @cache-nix-action/cache package that supports tar args.
  • The actions-toolkit directory was moved to the actions-toolkit branch.
• remove the option --skip-old-files from the tar command in the restore phase to overwrite files that should be overwritten with files from a cache (see the comment).
• don't try to restore the existing /nix/store files.
• add the save-always input introduced in the @actions/cache v4.