always extend link_exe with link flags

otherwise custom link-exe templates do not work

src/dds/toolchain/from_json.cpp

             assert(false && "No link-exe command");
             std::terminate();
         }
-        extend(ret, get_link_flags());
         return ret;
     });
+    extend(tc.link_exe, get_link_flags());
 
     tc.tty_flags = read_opt(tty_flags, [&]() -> string_seq {
         if (!compiler_id) {