Update flake.lock

Update the nix flake lock file.

This fixes the outdated glibc version, as described here:
#2565 (comment)

I updated to a fixed revision from Jan 2023, to pull in an old version of ocaml-mdx.
This is a workaround for the following regressions in later versions of mdx:
realworldocaml/mdx#428

To update nix flake inputs to a specific revision, I ran:

    nix flake update --override-input nixpkgs github:NixOS/nixpkgs/<rev>