put flattened .#hydraJobs in .#checks

Garnix does not build the `hydraJobs` output
so move them all into the `checks` output,
adhering to the flake output schema
because garnix does not build nested attrsets.

flake.nix

@@ -393,6 +393,28 @@
         }) required;
       });
 
+      checks = let
+        # https://github.com/numtide/flake-utils/issues/121#issuecomment-2589899217
+        recurseIntoDeepAttrs = attrs:
+          lib.recurseIntoAttrs (lib.mapAttrs (_: v:
+            if builtins.typeOf v == "set" && !lib.isDerivation v
+            then recurseIntoDeepAttrs v
+            else v
+          ) attrs);
+      in lib.genAttrs supportedSystems (system:
+        utils.lib.flattenTree (
+          recurseIntoDeepAttrs flake.ciJobs.${system}
+          // {
+            inherit (
+              let pkgs = self.legacyPackages.${system}; in
+              pkgs.callPackages iohkNix.utils.ciJobsAggregates {
+                ciJobs = lib.mapAttrs (_: lib.getAttr "required") flake.ciJobs.${system};
+              }
+            ) required;
+          }
+        )
+      );
+
       # allows precise paths (avoid fallbacks) with nix build/eval:
       outputs = self;