Optionally use gemini alias in the gemini_helpers.

  Order of preference is
      1. alias if use_alias is true
      2. binfs path
      3. which gemini
      4 'gemini' string

b: 489539893
Change-Id: I0f5258088bfc2e0bb1c4df7a619e1e6f8e96ba32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7635458
Commit-Queue: Jyothi Mathew <jyomathew@google.com>
Reviewed-by: James Woo <jewoo@google.com>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1594773}
NOKEYCHECK=True
GitOrigin-RevId: 9a4b52456da2c3829ab1ee5f3f58037e018482e8