test: remove reliance on in-tree deps/undici

Remove the dependency on the in-tree copy of Undici in `deps/undici`
from `parallel/test-inspector-network-fetch`. For downstream rebuilders
of Node.js using an externalized Undici it is not uncommon to remove
`deps/undici` to ensure that the build is not using the in-tree copy.

PR-URL: https://github.com/nodejs/node/pull/58866
Refs: https://github.com/nodejs/node/issues/58865
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-inspector-network-fetch.js

@@ -1,4 +1,4 @@
-// Flags: --inspect=0 --experimental-network-inspection
+// Flags: --inspect=0 --experimental-network-inspection --expose-internals
 'use strict';
 const common = require('../common');
 
@@ -12,8 +12,8 @@ const https = require('node:https');
 const inspector = require('node:inspector/promises');
 
 // Disable certificate validation for the global fetch.
-const undici = require('../../deps/undici/src/index.js');
-undici.setGlobalDispatcher(new undici.Agent({
+const undici = require('internal/deps/undici/undici');
+undici.setGlobalDispatcher(new undici.EnvHttpProxyAgent({
   connect: {
     rejectUnauthorized: false,
   },