nixos/shellhub-agent: use package internally, avoiding it in PATH

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
(cherry picked from commit fcb69a8583)

nixos/modules/services/networking/shellhub-agent.nix

@@ -95,8 +95,6 @@ in
         ExecStart = "${cfg.package}/bin/agent";
       };
     };
-
-    environment.systemPackages = [ cfg.package ];
   };
 }