Eval tests should be executed with the lowest supported Nix version (2.3)

[sternenseemann]

I ran into the first instance of path interpolation syntax slipping into nixpkgs today: https://github.com/NixOS/nixpkgs/pull/188466. Our lib/minver.nix prescribes that we support everything from 2.3 upwards.

Without CI to catch it, it is quite understandable that contributors accidentally introduce such changes (who remembers the changelog precisely?). Running the eval checks on the oldest supported Nix version should be enough, as any accepted Nix expressions under 2.3 should be forwards compatible.

[ghost]

I made this mistake too: https://github.com/NixOS/nixpkgs/pull/210207

OfBorg really needs to check this.

[SuperSandro2000]

This would also remove all performance optimizations done since the minimum supported versions.

[infinisil]

This is now implemented in Nixpkgs directly: https://github.com/NixOS/nixpkgs/commit/013acf2396f451c73b6cd6b0148df21cbaa4c165

[ShamrockLee]

Here is the explanation by @infinisil about the minimum supported Nix version on NixOS Discourse:

https://discourse.nixos.org/t/when-is-a-nix-builtin-function-builtins-considered-ready-to-be-used-inside-nixpkgs/35369/2