phpfpm service: don't use private /tmp

This breaks local PostgreSQL connections.

nixos/modules/services/web-servers/phpfpm/default.nix

···
           cfgFile = fpmCfgFile pool poolConfig;
         in {
           Slice = "phpfpm.slice";
-          PrivateTmp = true;
           PrivateDevices = true;
           ProtectSystem = "full";
           ProtectHome = true;