backwards: Replace nix-gc-env with nh clean

`nix.gc` run as root cannot prune generations under
`~/.local/state/nix/profiles` (NixOS/nix#8508), and `nix-gc-env` only
looks in `/nix/var/nix/profiles`. Home-manager generations therefore
accumulated since August 2024 and filled the disk.

`nh clean all` enumerates users and walks their per-user profiles too.
It replaces `nix.gc.automatic`, which cannot be enabled alongside it.

hosts/backwards/modules/boot/default.nix

@@ -1,5 +1,4 @@
 {
-  inputs,
   lib,
   config,
   ...
@@ -8,8 +7,6 @@ let
   configurationLimit = config.mod.gc.configurationLimit;
 in
 {
-  imports = [ inputs.nix-gc-env.nixosModules.default ];
-
   options = {
     mod.gc = {
       configurationLimit = lib.mkOption {
@@ -21,12 +18,17 @@ in
   };
 
   config = {
-    nix.gc = {
+    # `nix.gc` cannot prune generations under `~/.local/state/nix/profiles`
-      automatic = true;
+    # when run as root (NixOS/nix#8508), so home-manager generations piled up.
-      dates = "weekly";
+    # `nh clean` walks those per-user profiles too.
+    programs.nh = {
+      enable = true;
 
-      # `delete_generations` added by nix-gc-env
+      clean = {
-      delete_generations = "+${builtins.toString configurationLimit}";
+        enable = true;
+        dates = "weekly";
+        extraArgs = "--keep ${builtins.toString configurationLimit}";
+      };
     };
 
     boot = {