Take a look at nix-shell scripts. Much simpler, lightweight and fast. Start of a build script from a C project of mine:

  #!/usr/bin/env nix-shell
  #! nix-shell -i fish --pure
  #! nix-shell -I https://github.com/NixOS/nixpkgs/archive/4ecab3273592f27479a583fb6d975d4aba3486fe.tar.gz
  #! nix-shell --packages fish gcc gnumake gmp git cacert
  ...build script goes here...

-i fish: I am using fish shell.

--pure: Environment is as pure as it can reasonably get.

--packages: Packages from nixpkgs to be made available in the environment.

-I <github_commit_hash>: pinned to a specific git commit of nixpkgs. This script will give you the exact same environment with the exact same packages with exact same dependencies forever.

You can drop into a shell with the exact same configuration by using the cli command nix-shell with the same options.

Admittedly this is not as declarative as using flakes, since it's a script, but hey, I'm lazy, still didn't sit down to learn them once and for all.

Reference: https://nixos.org/manual/nix/stable/command-ref/nix-shell