Provide non-flake solution by pinning in shell.nix
Invoking `nix-shell` makes shell.nix not consume the flake. Importing
`<nixpkgs>` is dependent on the user's current nixpkgs channel. This is not
reproducible.
Fix this by pinning nixpkgs to the same revision of the flake input.
Ioannis Eleftheriou
Vera Aguilera Puerto