tadpole: Use option for gc configuration limit

hosts/tadpole/modules/boot/default.nix

@@ -1,14 +1,27 @@
-{ inputs, config, ... }:
+{ inputs, lib, config, ... }:
+let
+  configurationLimit = config.mod.gc.configurationLimit;
+in
 {
   imports = [ inputs.nix-gc-env.nixosModules.default ];
 
+  options = {
+    mod.gc = {
+      configurationLimit = lib.mkOption {
+        type = lib.types.int;
+        default = 10;
+        description = "number of configuration generations to keep";
+      };
+    };
+  };
+
   config = {
     nix.gc = {
       automatic = true;
       dates = "weekly";
 
       # `delete_generations` added by nix-gc-env
-      delete_generations = "+${builtins.toString config.boot.loader.systemd-boot.configurationLimit}";
+      delete_generations = "+${builtins.toString configurationLimit}";
     };
 
     boot = {
@@ -17,7 +30,7 @@
           enable = true;
           device = "/dev/sda";
 
-          configurationLimit = 10;
+          inherit configurationLimit;
         };
       };
     };

hosts/tadpole/modules/default.nix

@@ -8,6 +8,8 @@ in
 
   config = {
     mod = {
+      gc.configurationLimit = 10;
+
       ssh.enable = true;
       nginx.enable = true;