helmsman: only build helsman subPackage

helmsman drv would otherwise try to build the following subPackages:
https://github.com/Praqma/helmsman/tree/dadf5fa910fb25e48b617cb71f7f380d53970ea1/internal

(cherry picked from commit 19c58a0)

pkgs/by-name/he/helmsman/package.nix

@@ -16,6 +16,8 @@ buildGoModule rec {
     sha256 = "sha256-u/Fj3A81hH7i1yTg+kcqCPrwEkj0cyhZvNzRYURDoZU=";
   };
 
+  subPackages = [ "cmd/helmsman" ];
+
   vendorHash = "sha256-3eIMMKMvRzOSMvufETR9H1PnPDeEc+su8UuvbQJZ7kI=";
 
   doCheck = false;