Lazy binaries, that install the underlying command on invocation (the missing part is a gc root)

mkLazyBin.nix

with import <nixpkgs> {};
let
  mkLazyBin = { pkg, bin }:
    writeTextFile {
      name = bin;
      executable = true;
      text = ''
        #!/bin/sh
        [[ -d ${builtins.unsafeDiscardStringContext (pkg.outPath)} ]] || nix build --no-link ${builtins.unsafeDiscardStringContext (pkg.drvPath)}
        ${builtins.unsafeDiscardStringContext (pkg.outPath)}/bin/${bin}
      '';
    };
in mkLazyBin {
  pkg = hello;
  bin = "hello";
}