Update gemini_helper to prefer .ps1 over .js exts.

Without this the script defaults to returning hardcoded 'gemini'.

Bug: b:479940097
Change-Id: I822873a866470addc674ff9f89fd1aec29751b99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7679994
Commit-Queue: Jyothi Mathew <jyomathew@google.com>
Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: James Woo <jewoo@google.com>
Cr-Commit-Position: refs/heads/main@{#1602177}
NOKEYCHECK=True
GitOrigin-RevId: 3ab2461b0e04e5b7df90ef82c32ef31d551f602d