It's coming from pkgs.mkShell.

To read how that happens, open up pkgs/top-level/all-packages.nix, find mkShell. That gets you to pkgs/build-support/mkshell/default.nix.

Inside pkgs/build-support/mkshell/default.nix, note how the return value is stdenv.mkDerivation. (that stdenv is the key).

Consider also mkShellNoCC for your use case. Note how that definition in pkgs/top-level/all-packages.nix does this:

mkShellNoCC = mkShell.override { stdenv = stdenvNoCC; };

You can do similarly to override the stdenv with whatever's appropriate for your usecase.

In reply to @mr-qubo:matrix.org
When I do e.g. nix shell 'nixpkgs#htop' only htop is added to path. But if I create flake with devShell and htop in buildInputs, and use nix develop dependencies are added into the PATH as well, like with nix-shell -p htop. Is it possible to have this work as nix shell?
Also, as a second question, is it possible to group multiple packages and link all bins in one path, so only one entry in PATH is added instead of separate entry for each package?