process: hide NodeEnvironmentFlagsSet's add function

This makes sure that the `add` function is not visible by default
when inspecting `process.allowedNodeEnvironmentFlags`.

PR-URL: #28206
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/internal/process/per_thread.js

@@ -263,7 +263,9 @@ function buildAllowedFlags() {
 
       // The super constructor consumes `add`, but
       // disallow any future adds.
-      this.add = () => this;
+      Object.defineProperty(this, 'add', {
+        value: () => this
+      });
     }
 
     delete() {