Respect command registrations in plugins.

doc/manual/src/release-notes/rl-2.4.md

@@ -5,3 +5,4 @@
     to any command, including a subcommand to `nix`. For example,
     `nix-instantiate default.nix --plugin-files ""` must now become
     `nix-instantiate --plugin-files "" default.nix`.
+  - Plugins that add new `nix` subcommands are now actually respected.

src/libutil/args.cc

@@ -306,8 +306,8 @@ Strings argvToStrings(int argc, char * * argv)
     return args;
 }
 
-MultiCommand::MultiCommand(const Commands & commands)
-    : commands(commands)
+MultiCommand::MultiCommand(const Commands & commands_)
+    : commands(commands_)
 {
     expectArgs({
         .label = "subcommand",

src/nix/main.cc

@@ -159,6 +159,12 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
           #include "nix.md"
           ;
     }
+
+    // Plugins may add new subcommands.
+    void pluginsInited() override
+    {
+        commands = RegisterCommand::getCommandsFor({});
+    }
 };
 
 static void showHelp(std::vector<std::string> subcommand)