Work around gcc warning

This shuts up a 300-line warning that includes

  /nix/store/fg7ass3a5m5pgl26qzfdniicbwbgzccy-gcc-13.2.0/include/c++/13.2.0/bits/stl_tree.h:182:25: warning: ‘*(std::_Rb_tree_header*)((char*)&<unnamed> + offsetof(nix::value_type, nix::DerivedPath::<unnamed>.std::variant<nix::DerivedPathOpaque, nix::DerivedPathBuilt>::<unnamed>.std::__detail::__variant::_Variant_base<nix::DerivedPathOpaque, nix::DerivedPathBuilt>::<unnamed>.std::__detail::__variant::_Move_assign_base<false, nix::DerivedPathOpaque, nix::DerivedPathBuilt>::<unnamed>.std::__detail::__variant::_Copy_assign_base<false, nix::DerivedPathOpaque, nix::DerivedPathBuilt>::<unnamed>.std::__detail::__variant::_Move_ctor_base<false, nix::DerivedPathOpaque, nix::DerivedPathBuilt>::<unnamed>.std::__detail::__variant::_Copy_ctor_base<false, nix::DerivedPathOpaque, nix::DerivedPathBuilt>::<unnamed>.std::__detail::__variant::_Variant_storage<false, nix::DerivedPathOpaque, nix::DerivedPathBuilt>::_M_u) + 24).std::_Rb_tree_header::_M_header.std::_Rb_tree_node_base::_M_parent’ may be used uninitialized [-Wmaybe-uninitialized]
    182 |       if (__x._M_header._M_parent != nullptr)
        |           ~~~~~~~~~~~~~~^~~~~~~~~

Author: edolstra

src/nix/flake.cc

@@ -643,10 +643,11 @@ struct CmdFlakeCheck : FlakeCommand
                                             fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
                                             *attr2.value, attr2.pos);
                                         if (drvPath && attr_name == settings.thisSystem.get()) {
-                                            drvPaths.push_back(DerivedPath::Built {
+                                            auto path = DerivedPath::Built {
                                                 .drvPath = makeConstantStorePathRef(*drvPath),
                                                 .outputs = OutputsSpec::All { },
-                                            });
+                                            };
+                                            drvPaths.push_back(std::move(path));
                                         }
                                     }
                                 }