default.nix

{ stdenv, musl, ... }:
stdenv.mkDerivation {
  name = "env-friendly-1.0.0";

  unpackPhase = ''
    cp -v ${./Makefile} Makefile
    cp -v ${./env.c} env.c
  '';

  CFLAGS = "-isystem ${musl}/include -B${musl}/lib -L${musl}/lib";
  LDFLAGS = "-static -Wl,--gc-sections";

  meta = with stdenv.lib; {
    description = "An environmentally friendly replacement for /usr/bin/env";
    license = licenses.cc0;
    maintainers = [ "Nathan Zadoks <[email protected]>" ];
    platforms = stdenv.lib.platforms.linux;
  };
}

env.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
int main(int argc, char *argv[]) {
  if (argc < 2) {
    fprintf(stderr, "%s: no command specified\n", argv[0]);
    exit(-1);
  }
  execvp(argv[1], &argv[1]);
  char* err = strerror(errno);
  fprintf(stderr, "%s: %s: %s", argv[0], argv[1], err);
  exit(127);
}

Makefile

all: env
install: env
	install -D env $(out)/bin/env

module.nix

{ config, pkgs, lib, ... }: with lib; let
  env-friendly = callPackage ./. {};
in {
  system.activationScripts.usrbinenv = mkForce ''
    mkdir -m 0755 -p /usr/bin
    ln -sfn ${env-friendly}/bin/env /usr/bin/.env.tmp
    mv /usr/bin/.env.tmp /usr/bin/env # atomically replace /usr/bin/env
  '';
}