Kernel: Use MUST+ Vector::try_prepend instead of Vector::prepend

In preparation for disabling Vector::prepend from being available
during the compilation of the Kernel.

Kernel/CommandLine.cpp

@@ -261,7 +261,7 @@ NonnullOwnPtrVector<KString> CommandLine::userspace_init_args() const
 
     auto init_args = lookup("init_args"sv).value_or(""sv).split_view(';');
     if (!init_args.is_empty())
-        args.prepend(KString::must_create(userspace_init()));
+        MUST(args.try_prepend(KString::must_create(userspace_init())));
     for (auto& init_arg : init_args)
         args.append(KString::must_create(init_arg));
     return args;