Nix: checks: add all packages

Author: malikwirin

contrib/nix/flake.nix

@@ -213,9 +213,8 @@
         formatter = treefmt.config.build.wrapper;
 
         checks = {
-          inherit stacks-core;
           formatting = treefmt.config.build.check self;
-        };
+        } // self.packages."${system}";
 
         devShells.default = craneLib.devShell {
           RUSTFMT = "${toolchain}/bin/rustfmt";