Add GOOGLE_GENAI_USE_VERTEXAI variable setting to make workflow work

[huan]

Fix the action error:

Installing Gemini CLI from npm: @google/gemini-cli@latest
Verifying installation:
0.13.0
Run set -euo pipefail
Error: Please set an Auth method in your /home/runner/.gemini/settings.json or specify one of the following environment variables before running: GEMINI_API_KEY, GOOGLE_GENAI_USE_VERTEXAI, GOOGLE_GENAI_USE_GCA

See: https://github.com/huan/ai-student-assistant/actions/runs/19178287968/job/54828468408#step:5:410

Fix #388

I ran into this issue today at DevFest Workshop Secure GitHub Workflow with Gemini CLI by Tianzi Cai & Rody Davis, Google.

[Copilot]

Pull Request Overview

This PR adds automatic configuration of the GOOGLE_GENAI_USE_VERTEXAI variable during the Workload Identity Federation setup process. This ensures that workflows using Vertex AI authentication will have the required variable set correctly.

• Adds GOOGLE_GENAI_USE_VERTEXAI variable to the automated GitHub variable setup when the gh CLI is available

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[MJjainam]

@gemini-cli /review

[gemini-cli]

🤖 Hi @MJjainam, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[gemini-cli]

📋 Review Summary

This Pull Request introduces a necessary fix by adding the GOOGLE_GENAI_USE_VERTEXAI variable to the setup script, which resolves an authentication error. The change is well-contained and directly addresses the reported issue.

🔍 General Feedback

• The inclusion of echo statements for the newly set variable provides good visibility into the script's actions.
• The overall approach aligns with the existing script's structure and conventions.

[gemini-cli]

🟡 Consider clarifying how to 'use GOOGLE_GENAI_USE_GCA'. The current phrasing might imply setting GOOGLE_GENAI_USE_GCA to true, but it's not explicitly stated as it is for GOOGLE_GENAI_USE_VERTEXAI. Perhaps something like 'or set GOOGLE_GENAI_USE_GCA to 'true' for Code Assist'.

Suggested change

	echo " (Set to 'true' to use Vertex AI, or use GOOGLE_GENAI_USE_GCA for Code Assist)"
	echo " (Set to 'true' to use Vertex AI, or set GOOGLE_GENAI_USE_GCA to 'true' for Code Assist)"