Use GitHub "checks" to provide more detail than "status reports"

[MattSturgeon]

As discussed (#217 (comment)), it'd be useful to provide additional information via the "checks" API. Especially now that not all builds are reported.

• Example "check details" from Mergify: https://github.com/nix-community/nixvim/pull/1933/checks?check_run_id=27938204893
• GitHub docs: About status checks

Checks can also be used to provide errors associated with a specific line-number, however this would probably be more involved to implement than a markdown overview.