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.
(cherry picked from commit 2629a8d92c7b22dfe4c860cba4fa9806c4e7ee02)
Ioannis Eleftheriou
Vera Aguilera Puerto