test: fix test-repl-envvars

In 180f865, 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: nodejs#21451
Fixes: nodejs/build#1377
Refs: nodejs#25219

test/parallel/test-repl-envvars.js

@@ -36,7 +36,7 @@ const tests = [
 ];
 
 function run(test) {
-  const env = Object.assign({}, process.env, test.env);
+  const env = test.env;
   const expected = test.expected;
   const opts = {
     terminal: true,