nixos/iperf3: add package option

nixos/modules/services/networking/iperf3.nix

@@ -4,6 +4,7 @@ let
 
   api = {
     enable = mkEnableOption "iperf3 network throughput testing server";
+    package = mkPackageOption pkgs "iperf3" { };
     port = mkOption {
       type        = types.ints.u16;
       default     = 5201;
@@ -76,7 +77,7 @@ let
         CapabilityBoundingSet = "";
         NoNewPrivileges = true;
         ExecStart = ''
-          ${pkgs.iperf3}/bin/iperf \
+          ${lib.getExe cfg.package} \
             --server \
             --port ${toString cfg.port} \
             ${optionalString (cfg.affinity != null) "--affinity ${toString cfg.affinity}"} \

pkgs/tools/networking/iperf/3.nix

@@ -42,6 +42,7 @@ stdenv.mkDerivation rec {
     description = "Tool to measure IP bandwidth using UDP or TCP";
     platforms = lib.platforms.unix;
     license = lib.licenses.bsd3;
+    mainProgram = "iperf3";
     maintainers = with lib.maintainers; [ fpletz ];
   };
 }