fix build against nix 2.7.0

fix build after such commits as df552ff53e68dff8ca360adbdbea214ece1d08ee
and e862833ec662c1bffbe31b9a229147de391e801a

src/hydra-queue-runner/nar-extractor.cc

@@ -64,7 +64,7 @@ struct Extractor : ParseSink
         }
     }
 
-    void createSymlink(const Path & path, const string & target) override
+    void createSymlink(const Path & path, const std::string & target) override
     {
         members.insert_or_assign(prefix + path, NarMemberData { .type = FSAccessor::Type::tSymlink });
     }