nix repl needs to switch to the new syntax

[Ericson2314]

#9043 was not quite right, because it made the usage without the flakes experimental feature unconditionally have the old syntax. But if we're stabilizing the CLI first (which we are) we want nix repl to have the new syntax (consistent with other commands) regardless of whether the flakes feature is enabled.

Either we need to unconditionally switch to the new syntax, or we need to reintroduce repl-flake but make flakes imply it.

See my comments on that PR for details.