lib: run prepareMainThreadExecution for third_party_main

Treat `_third_party_main` like any other CJS entry point, as it
was done before 6967f91.

PR-URL: #26677
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: addaleax

lib/internal/main/run_third_party_main.js

@@ -1,9 +1,13 @@
 'use strict';
 
-// Legacy _third_party_main.js support
+const {
+  prepareMainThreadExecution
+} = require('internal/bootstrap/pre_execution');
 
+prepareMainThreadExecution();
 markBootstrapComplete();
 
+// Legacy _third_party_main.js support
 process.nextTick(() => {
   require('_third_party_main');
 });