Default to and correctly use hermetic gCLI

Updates the prompt eval-related code to default to using the hermetic
CIPD-provided Gemini CLI binary and to properly pass that through to
code that needs it. Previously, even if the hermetic binary was
supposed to be used, it would largely be ignored due to calls to
gemini_helpers.py.

Further refactoring to have a single source of truth for the binary
and version are tracked in b/508717027, but this should at least get
the tests happy for now.

Bug: b/508378930
Change-Id: Ibe1a7ced0397bc1701ab53edf4f91e35c7935e63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7808988
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1624670}
NOKEYCHECK=True
GitOrigin-RevId: 9d799605767b58e013e7e4288e1c2fb4f54b6899