flake/ci: fix lazy eval of dev partition

If the `ci` options are a set of options rather than a single option,
then the module system cannot lazily override it via `mkForce`.

In practice, this means that the dev partition gets evaluated strictly
when the module system "pushes down" the `ci` definitions to its
options.

Instead, we must ensure that each attribute listed in `partitionedAttrs`
corresponds to an actual option which can be directly defined, with no
"pushing down" required.

Fixes #3532

(cherry picked from commit 67785f9577)

flake/ci.nix

@@ -4,9 +4,13 @@ let
     mapAttrs
     types
     ;
-
+in
-  buildbotOpt = lib.mkOption {
+{
-    type = types.lazyAttrsOf types.package;
+  perSystem = {
+    # per-system CI options
+    options.ci = {
+      buildbot = lib.mkOption {
+        type = types.lazyAttrsOf types.raw;
         default = { };
         description = ''
           A set of tests for [buildbot] to run.
@@ -14,28 +18,26 @@ let
           [buildbot]: https://buildbot.nix-community.org
         '';
       };
-in
-{
-  perSystem = {
-    # Declare per-system CI options
-    options.ci = {
-      buildbot = buildbotOpt;
     };
   };
 
   flake = {
-    # Declare top-level CI options
+    # top-level CI option
-    options.ci = {
+    #
-      buildbot = lib.mkOption {
+    # NOTE:
-        type = types.lazyAttrsOf buildbotOpt.type;
+    #   This must be an actual option, NOT a set of options.
+    #   Otherwise, flake partitions will not be lazy.
+    options.ci = lib.mkOption {
+      type = types.lazyAttrsOf (types.lazyAttrsOf types.raw);
       default = { };
       description = ''
-          See `perSystem.ci.buildbot` for description and examples.
+        Outputs related to CI.
+
+        Usually defined via the `perSystem.ci` options.
       '';
     };
-    };
 
-    # Transpose per-system CI outputs to the top-level
+    # Transpose per-system definitions to the top-level
     config.ci = {
       buildbot = mapAttrs (_: sysCfg: sysCfg.ci.buildbot) config.allSystems;
     };