Adopt kradalby/flake-checks: flake.nix exposes build, gotest,
golangci-lint and formatting checks over one shared, fileset-filtered
source set, and nix-checks.yml gates each with
nix build .#checks.<system>.<name>. Drop test.yml and lint.yml —
gotestsum, golangci-lint and prettier are now those checks. golangci-lint
runs full-tree; Go formatting stays in it while formatting adds prettier.
hscontrol/servertest is too slow and timing-sensitive for the sandboxed
gotest check, so servertest.yml runs it in the devShell instead.
Kristoffer Dalby