prometheus-postfix-exporter: buildFlags -> tags

pkgs/servers/monitoring/prometheus/postfix-exporter.nix

···
 
   nativeBuildInputs = optional withSystemdSupport makeWrapper;
   buildInputs = optional withSystemdSupport systemd;
-  buildFlags = optional (!withSystemdSupport) "-tags nosystemd";
+  tags = optional (!withSystemdSupport) "nosystemd";
 
   goDeps = ./postfix-exporter-deps.nix;
   extraSrcs = optionals withSystemdSupport [