Installs `rustc` from nixpkgs when already installed

[pseudoparenchymatous]

I am using fenix as my rust overlay; installed it in my dev-shell using nativeBuildInputs. But when I run nix build, it reinstalls the toolchain (i.e. rustc) from nixpkgs.

And if I remove nixpkgs from my inputs, nix build behaves as expected and builds the crate.