fix build against nix 2.7.0

fix build after such commits as df552ff53e68dff8ca360adbdbea214ece1d08ee
and e862833ec662c1bffbe31b9a229147de391e801a

src/libhydra/db.hh

@@ -18,7 +18,7 @@ struct Connection : pqxx::connection
         std::string upper_prefix = "DBI:Pg:";
 
         if (hasPrefix(s, lower_prefix) || hasPrefix(s, upper_prefix)) {
-            return concatStringsSep(" ", tokenizeString<Strings>(string(s, lower_prefix.size()), ";"));
+            return concatStringsSep(" ", tokenizeString<Strings>(std::string(s, lower_prefix.size()), ";"));
         }
 
         throw Error("$HYDRA_DBI does not denote a PostgreSQL database");