Adds top-level and per-system `ci.buildbot` option for defining tests to
build on buildbot.
These are exposed as the `ci.buildbot.«system»` flake output.
(cherry picked from commit e18d3fb280)
This is much faster than running `nix fmt`.
Running treefmt directly will use the treefmt installed in the current
shell. Running `nix fmt` will first re-evaluate the flake.
This removes the need for end-users to manually set
`nixvim.inputs.devshell.follows = ""` (etc)
We offload evaluation of some of our flake modules into a `dev`
partition submodule.
- When its not needed, this submodule is not evaluated.
- When it is needed, it fetches extra inputs from `flake/dev/flake.nix`
as part of evaluating the submodule.
See https://flake.parts/options/flake-parts-partitions.html
