chore: Use .built/HOME in Nix (fixes #1876)

[vorburger]

Fixes #1876.

This is a sub-set of the (obsolete) #1877, which was a sub-set of #1873.

@dotdoom LGTY?

[gemini-code-assist]

Summary of Changes

Hello @vorburger, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses issue #1876 by refining the Nix build environment configuration. It modifies how the HOME directory is handled during the build process, directing it to a dedicated .built/HOME path. This change, coupled with updates to .bazelignore and .gitignore, aims to improve build isolation, prevent unintended file tracking, and ensure a cleaner build output by properly managing temporary and generated files within the Nix environment.

Highlights

• Nix Build Environment: The HOME environment variable in the Nix build process is now explicitly set to .built/HOME instead of $TMPDIR, and this directory is created to ensure proper isolation for build-related temporary files.
• Build System Ignores: The .bazelignore and .gitignore files have been updated to include the new .built directory, preventing these generated files from being tracked by Bazel and Git.
• Build Hooks Integration: Standard runHook calls (preBuild, postBuild, preInstall, postInstall) have been added to the buildPhase and installPhase in flake.nix, providing more flexibility for custom build steps.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

This pull request correctly addresses an issue with the Nix build by redirecting the HOME directory to a local path within the build environment. This is a good approach to improve build reliability. The related changes in .gitignore and .bazelignore are also appropriate. I have one suggestion in flake.nix to make the script more robust by using an absolute path.