You may be interested in Flakes[0], which locks your nixpkgs (and other "inputs") to a specific known version.

Obligatory disclaimer, they're still in beta, but everyone's using them anyway. There's some ongoing work[1] before they're officially released.

[0] https://nixos.wiki/wiki/Flakes [1] https://www.youtube.com/watch?v=JE-vLFMTXxM