`nix repl` fails when arg path includes an "@" symbol

Describe the bug

When calling nix repl --arg flakePath "/some/path/to/@directory", the command fails with
an error stating "path has a trailing slash". This seems somewhat similar to #3189 where Nix
may simply be over-restrictive of what characters can allow in a path. I found this issue a
few weeks ago when trying to make use of flake-utils-plus's
repl helper within a directory that had an @ in the file path. As it turned out, this problem
happened with nix repl alone.

Steps To Reproduce

1. Run nix repl --arg flakePath "/some/path/to/@directory"
2. You will see the following output.

$ nix repl --arg flakePath "/some/path/to/@directory"
error: path has a trailing slash

       at «string»:1:15:

            1| /some/path/to/@directory
             |               ^

Expected behavior

Nix should allow any valid path character within paths.

I understand that there may be quite a lot to consider for changing how paths work in Nix,
so I would be happy to settle with adding support for @ at the moment. However, it would
be a good thing for Nix to allow non-ascii characters in general.

nix-env --version output

nix-env (Nix) 2.8.1

Additional context

I don't believe it's necessary to resolve this issue since the problem seems to lie within Nix itself, but here is the line in flake-utils-plus's repl helper that calls nix repl --arg ...: https://github.com/gytis-ivaskevicius/flake-utils-plus/blob/2bf0f91643c2e5ae38c1b26893ac2927ac9bd82a/lib/overlay.nix#L19

No Nix source file or test is named. Start by reproducing the nix repl --arg flakePath "/some/path/to/@directory" command and trace how nix repl parses the argument path. Done means a valid path containing @ is accepted without the trailing-slash error.