nixos: add backup file extension and verbosity options

nixos/default.nix

@@ -23,6 +23,14 @@ let
     };
   });
 
+  serviceEnvironment =
+    optionalAttrs (cfg.backupFileExtension != null) {
+      HOME_MANAGER_BACKUP_EXT = cfg.backupFileExtension;
+    }
+    // optionalAttrs cfg.verbose {
+      VERBOSE = "1";
+    };
+
 in
 
 {
@@ -33,6 +41,18 @@ in
         <option>users.users.&lt;name?&gt;.packages</option> option.
       '';
 
+      backupFileExtension = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = "backup";
+        description = ''
+          On activation move existing files by appending the given
+          file extension rather than exiting with an error.
+        '';
+      };
+
+      verbose = mkEnableOption "verbose output on activation";
+
       users = mkOption {
         type = types.attrsOf hmModule;
         default = {};
@@ -70,6 +90,8 @@ in
           wants = [ "nix-daemon.socket" ];
           after = [ "nix-daemon.socket" ];
 
+          environment = serviceEnvironment;
+
           serviceConfig = {
             User = usercfg.home.username;
             Type = "oneshot";