Dependabot-style updates for nix flakes / `flake.lock`

[cdata]

See discussion in https://github.com/commontoolsinc/system/pull/126#discussion_r1733482272

[Mearman]

References:

• https://github.com/dependabot/dependabot-core/issues/7340
• https://docs.renovatebot.com/modules/manager/nix/
• https://github.com/DeterminateSystems/update-flake-lock