flake/ci: add ci.buildbot output

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)

flake/ci.nix

@@ -0,0 +1,43 @@
+{ lib, config, ... }:
+let
+  inherit (lib)
+    mapAttrs
+    types
+    ;
+
+  buildbotOpt = lib.mkOption {
+    type = types.lazyAttrsOf types.package;
+    default = { };
+    description = ''
+      A set of tests for [buildbot] to run.
+
+      [buildbot]: https://buildbot.nix-community.org
+    '';
+  };
+in
+{
+  perSystem = {
+    # Declare per-system CI options
+    options.ci = {
+      buildbot = buildbotOpt;
+    };
+  };
+
+  flake = {
+    # Declare top-level CI options
+    options.ci = {
+      buildbot = lib.mkOption {
+        type = types.lazyAttrsOf buildbotOpt.type;
+        default = { };
+        description = ''
+          See `perSystem.ci.buildbot` for description and examples.
+        '';
+      };
+    };
+
+    # Transpose per-system CI outputs to the top-level
+    config.ci = {
+      buildbot = mapAttrs (_: sysCfg: sysCfg.ci.buildbot) config.allSystems;
+    };
+  };
+}