Select a nixpkgs revision?

[endgame]

It's great that you're using nix to ensure a reproducible software environment for your research, but your default.nix depends on whatever version of nixpkgs you currently have selected.

You can fix this by selecting a revision of https://github.com/NixOS/nixpkgs in default.nix. Here's an example from a recent nixpkgs master:

with import (builtins.fetchTarball {
  url = "https://github.com/NixOS/nixpkgs/archive/0a8d74d95761c32147a72f16954455e64f33ba13.tar.gz";
  sha256 = "09qsbbfqx5ar8grfrfzjl3nm8mgk8cv1pi8s64mhjr7dh6mp7h64";
});