test: fix test-repl-envvars

In 180f8650, the test was changed
so that the `env` argument of `createInternalRepl()` also contained
external environment variables, because keeping them can be necessary
for spawning processes on some systems.

However, this test does not spawn new processes, and relies on the
fact that the environment variables it tests are not already set
(and fails otherwise); therefore, reverting to the original state
should fix this.

Fixes: https://github.com/nodejs/node/issues/21451
Fixes: https://github.com/nodejs/build/issues/1377
Refs: https://github.com/nodejs/node/pull/25219

PR-URL: https://github.com/nodejs/node/pull/25226


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>