Use parent envrc / default.nix.

.envrc

@@ -1 +0,0 @@
-use_nix

default.nix

@@ -1,27 +0,0 @@
-let
-  pkgs = import <nixpkgs> {
-    overlays = [
-      (import (builtins.fetchTarball
-        "https://github.com/oxalica/rust-overlay/archive/master.tar.gz"))
-    ];
-  };
-
-  rust = pkgs.rust-bin.stable.latest.rust.override {
-    extensions = [ "rust-src" ];
-  };
-
-in with pkgs;
-pkgs.mkShell rec {
-  name = "rust";
-  buildInputs = [
-    openssl
-    pkg-config
-    cargo
-    rust
-    rustfmt
-    rust-analyzer
-    wasm-pack
-    wasm-bindgen-cli
-    nodePackages.rollup
-  ];
-}