netns: don't repeat cli definitions

1. Saves some code.
2. Guarantees that the netns and no-netns cli defs are always in sync.

modules/nix-bitcoin-services.nix

@@ -55,4 +55,11 @@ with lib;
     set -eo pipefail
     ${src}
   '';
+
+  cliExec = mkOption {
+    # Used by netns-isolation to execute the cli in the service's private netns
+    internal = true;
+    type = types.str;
+    default = "exec";
+  };
 }