update-flake-inputs: init

[l0b0]

Description

The more complex a flake, the less likely that nix flake update will Just Work™. This adds a module to do the grunt work automatically for any number of repos with a flake.nix and flake.lock on a schedule:

1. Make sure that flake.nix is in a clean state in the Git repository,
   otherwise skip the directory.
2. Make sure nothing is staged in the Git repository, otherwise skip the
   directory.
3. Update a single input at a time. For each input:
   1. If the input is already up to date, skip the rest of the steps.
   2. Make sure all the NixOS configurations and dev shells still build
      after each update (optionally with extra commands).
   3. Commit flake.lock (and only flake.lock) if all the build and extra
      commands pass.
   4. Revert flake.lock if any of the above fails.
4. Report errors about inputs which can't be updated after processing each
   directory.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News

[l0b0]

This is ending up being rather user-specific. I'll re-open if I can figure out a non-horrible way to make this more generic.