add a check

flake.nix

@@ -8,8 +8,17 @@
     let
       forAllSystems = nixpkgs.lib.genAttrs [
         "x86_64-linux"
-        "aarch64-linux"
       ];
+      # Discover child flake directories automatically
+      childFlakeDirs =
+        let
+          entries = builtins.readDir ./.;
+        in
+        builtins.filter (
+          name:
+          entries.${name} == "directory"
+          && builtins.pathExists (./. + "/${name}/flake.nix")
+        ) (builtins.attrNames entries);
     in
     {
       packages = forAllSystems (
@@ -37,5 +46,27 @@
           program = "${self.packages.${system}.update}/bin/update-pins";
         };
       });
+
+      checks = forAllSystems (
+        system:
+        builtins.listToAttrs (
+          map (
+            name:
+            let
+              lock = builtins.fromJSON (builtins.readFile (./. + "/${name}/flake.lock"));
+              nixpkgsNode = lock.nodes.${lock.nodes.root.inputs.nixpkgs};
+              nixpkgsSrc = builtins.fetchTarball {
+                url = "https://github.com/${nixpkgsNode.locked.owner}/${nixpkgsNode.locked.repo}/archive/${nixpkgsNode.locked.rev}.tar.gz";
+                sha256 = nixpkgsNode.locked.narHash;
+              };
+              pkgs = import nixpkgsSrc { inherit system; };
+            in
+            {
+              inherit name;
+              value = pkgs.matrix-synapse;
+            }
+          ) childFlakeDirs
+        )
+      );
     };
 }