nix-shell no longer respects NIX_PATH

[SuperSandro2000]

Describe the bug

When running NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/nixos-unstable.tar.gz command nix-shell --option nix-path "" -p bash I am only greeted with

error:
       … while calling the 'import' builtin
         at «string»:1:18:
            1| {...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (bash) ]; } ""
             |                  ^

       … while realising the context of a path

       … while calling the 'findFile' builtin
         at «string»:1:25:
            1| {...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (bash) ]; } ""
             |                         ^

       error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

but command nix-shell --option nix-path nixpkgs=https://github.com/SuperSandro2000/nixpkgs/archive/nixos-unstable.tar.gz -p bash works as expected

Steps To Reproduce

1. Run NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/nixos-unstable.tar.gz command nix-shell --option nix-path "" -p bash

Expected behavior

It should work as it did with 2.20 or 2.18.

nix-env --version output
nix-env (Nix) 2.21.0

Additional context

Add any other context about the problem here.

Priorities

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[thufschmitt]

Did it use to work?

This looks like the broken semantics “documented” in #9066

(not saying it's not an issue, just clarifying whether this is the same old bug or a real regression)

[SuperSandro2000]

Did it use to work?

As soon as I switch back to 2.20 or 2.18 by changing nix.package it starts to work again.

[thufschmitt]

Weird, I can reproduce that for every Nix version >= 2.4 (2.3 works, but that's just because it doesn't have the nix-path setting)

[nasrally]

I see identical behavior on 2.18.1.

NIX_PATH is completely ignored by nix-shell yet it is present in the environment variables, I can see it in /proc/PID/environ, defining the nix-path setting in either /etc/nix/nix.conf (through NixOS), $HOME/.config/nix/nix.conf (through HM) or --option nix-path nixpkgs=... DOES WORK.

Maybe important context that makes this unrelated — it started doing that after I disabled channels on NixOS with nix.channel.enable = false (because I want complete flakification)