nh: fixes and addition to warnings/assertions (#6470)

* nh: fix auto clean

- ~Fix `PATH` variable~
- Remove flake path assertion
- Add warning when HM's `nix.gc.automatic` is enabled

* nh: simplify logic for warning related to cfg.clean

---------

Co-authored-by: Nikita Pedorich <pedorich.n@gmail.com>

modules/programs/nh.nix

@@ -52,15 +52,11 @@ in {
   };
 
   config = {
-    warnings = lib.optionals
-      (osConfig != null && !(cfg.clean.enable -> !osConfig.nix.gc.automatic)) [
-        "programs.nh.clean.enable and nix.gc.automatic (system-wide in configuration.nix) are both enabled. Please use one or the other to avoid conflict."
-      ];
-
-    assertions = [{
-      assertion = (cfg.flake != null) -> !(lib.hasSuffix ".nix" cfg.flake);
-      message = "nh.flake must be a directory, not a nix file";
-    }];
+    warnings = (lib.optional
+      (cfg.clean.enable && osConfig != null && osConfig.nix.gc.automatic)
+      "programs.nh.clean.enable and nix.gc.automatic (system-wide in configuration.nix) are both enabled. Please use one or the other to avoid conflict.")
+      ++ (lib.optional (cfg.clean.enable && config.nix.gc.automatic)
+        "programs.nh.clean.enable and nix.gc.automatic (Home-Manager) are both enabled. Please use one or the other to avoid conflict.");
 
     home = lib.mkIf cfg.enable {
       packages = [ cfg.package ];