`nix shell` does not insert `PATH` items in command line order

[roberth]

Describe the bug

nix shell does not insert PATH items in command line order. Instead, it traverses them left to right prepending each, resulting in a reversal.

Steps To Reproduce

Apologies for the whacky example.

1. Use my nixpkgs branch with a customized hello package. (will link; doesn't matter much)
2. nix shell .#hello .#salve-mundi
3. hello

It invokes the latter package, printing SALVE MUNDI

Expected behavior

Prepend them to the existing PATH but in the same order as specified on the command line.

nix-env --version output

master 92611e6

Additional context

• CLI stabilization effort #7701

Priorities

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[cole-h]

I took a stab at this in #9648.