packet-sd: enable build on unix

pkgs/development/tools/packet-sd/default.nix

···
     description = "Prometheus service discovery for Equinix Metal";
     homepage = "https://github.com/packethost/prometheus-packet-sd";
     license = licenses.asl20;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = [ maintainers.andir ];
   };
 }