flake-parts: add 'description' to the flake check

[srid]

Adding description meta is useful in apps that inspect the flake and present the information to the user. I noticed that the treefmt package itself has a description:

But the flake check does not,

(Screenshots from https://github.com/juspay/nix-browser)

This PR resolves that.

CLI,

nix-repl> checks.aarch64-darwin.treefmt.meta.description
"Check that the project tree is formatted"

[srid]

Diff: https://github.com/numtide/treefmt-nix/pull/116/files?diff=unified&w=1

[srid]

Garnix fails for some reason, but nix flake check succeeds locally.

[zimbatm]

thanks!