pin nixpkgs version when running the test

[teto]

Description

running make all-tests calls: nix-shell --pure tests -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz -A run.neovim-plugin-config so you sometimes have to rebuild tests etc, people may see different results, it may succeed on a PR but fail once merged.

We should rely on the flakes to get a pinned nixpkgs (eventualoly via flake-compat https://github.com/edolstra/flake-compat/).

Also we could make clearer what values to pass to "TEST", e.g.: make test TEST=neovim-plugin-config

[stale[bot]]

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