US: Multiple nixpkgs versions as dependency for packages

[imincik]

Description

As a NGI Forge application contributor, I want to be able to choose from multiple versions of nixpkgs (stable, unstable, old-stable) for my package dependencies so that I can choose the one which fits the best for my application.

Acceptance criteria

• Given < some context >, when < some action is carried out >, then < set of observable outcomes should occur >.

Notes

We can extend pkgs to provide

• pkgs.mypkgs (rename to pkgs.forge-pkgs)

• pkgs.unstable

• pkgs.nixos-26.05

• pkgs.nixos-25.11