2.13.0 regression: fetchTarball + channel: URL

[Kha]

Describe the bug

The fetchTarball builtin (and possibly others?) does not accept a channel: URL anymore, quoted or not.

Steps To Reproduce

$ nix run github:NixOS/nix/2.13.0 -- eval --impure --expr "fetchTarball \"channel:nixos-22.11\""
error:
       … while calling the 'fetchTarball' builtin

         at «string»:1:1:

            1| fetchTarball "channel:nixos-22.11"
             | ^

       error: unable to download 'channel:nixos-22.11': URL using bad/illegal format or missing URL (3)
$ nix run github:NixOS/nix/2.12.0 -- eval --impure --expr "fetchTarball \"channel:nixos-22.11\""
"/nix/store/i63d7z75f27p6388hzksamvir7zv7rbl-source"
$ nix run github:NixOS/nix/2.13.0 -- eval --impure --expr "fetchTarball channel:nixos-22.11"
error:
       … while calling the 'fetchTarball' builtin

         at «string»:1:1:

            1| fetchTarball channel:nixos-22.11
             | ^

       error: unable to download 'channel:nixos-22.11': URL using bad/illegal format or missing URL (3)
$ nix run github:NixOS/nix/2.12.0 -- eval --impure --expr "fetchTarball channel:nixos-22.11"
"/nix/store/i63d7z75f27p6388hzksamvir7zv7rbl-source"

Expected behavior

There should be no regression.

nix-env --version output

nix-env (Nix) 2.13.0

Additional context

I should probably update this code to --override-input, but I thought I should still flag the issue.

Priorities

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