fix(ci): disable nightly release workflow on forks

[wtanaka]

The scheduled nightly release workflow fails on forks due to missing secrets. This change adds a condition to the 'release' job to ensure it only runs on the main repository, preventing it from failing on forks.

[gemini-code-assist[bot]]

[!NOTE] Gemini is unable to generate a summary for this pull request due to the file types involved not being currently supported.

[bdmorgan]

Hi @wtanaka, thank you so much for your contribution to Gemini CLI! We really appreciate the time and effort you've put into this.

We're making some updates to our contribution process to improve how we track and review changes. Please take a moment to review our recent discussion post: Improving Our Contribution Process & Introducing New Guidelines.

Key Update: Starting January 26, 2026, the Gemini CLI project will require all pull requests to be associated with an existing issue. Any pull requests not linked to an issue by that date will be automatically closed.

Thank you for your understanding and for being a part of our community!

[gemini-cli[bot]]

Hi there! Thank you for your contribution to Gemini CLI.

To improve our contribution process and better track changes, we now require all pull requests to be associated with an existing issue, as announced in our recent discussion and as detailed in our CONTRIBUTING.md.

This pull request is being closed because it is not currently linked to an issue. You can easily reopen this PR once you have linked it to an issue.

How to link an issue: Add a keyword followed by the issue number (e.g., Fixes #123) in the description of your pull request. For more details, see the GitHub Documentation.

Thank you for your understanding and for being a part of our community!