chore: Bump Nixpkgs from 25.05 to unstable, and use bazel_8 instead of bazelisk

[vorburger]

This is only a 1st step; do note how both the bazelisk package and the .bazelversion have not been removed, yet.

Unfortunately they are still required, for now - because various scripts have not been cleaned-up, yet. Contributions welcome!

Relates to #1709 and (indirectly) also #1730 etc.

@dotdoom FYI

[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 performs a significant update to the project's Nix environment by bumping the Nixpkgs channel from the stable "nixos-25.05" to the "nixos-unstable" branch. Concurrently, it begins the process of migrating the Bazel build system from relying on "bazelisk" to directly utilizing the "bazel_8" package. This change is explicitly marked as an initial phase, with subsequent work planned to fully deprecate and remove "bazelisk" from the project's configuration and scripts.

Highlights

• Nixpkgs Channel Update: The Nixpkgs channel has been updated from "nixos-25.05" to "nixos-unstable" in both "flake.nix" and "flake.lock", reflecting a move to the latest development branch.
• Bazel Version Management Transition: This pull request initiates the transition from using "bazelisk" to directly using the "bazel_8" package. "bazel_8" has been added to the development shell, and one build command has been updated to use "bazel" instead of "bazelisk".
• Incremental Change: This change is explicitly noted as a first step in a larger migration. The "bazelisk" package and the ".bazelversion" file are still present, indicating that further cleanup of related scripts and configurations is planned for future changes.

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[gemini-code-assist]

Code Review

This pull request updates Nixpkgs from version 25.05 to unstable and initiates the transition from bazelisk to bazel_8. The changes are clear and align with the description, which correctly states this is an initial step. The move to nixos-unstable is a significant change that provides access to newer packages, and the use of a lock file ensures builds remain reproducible. I have one suggestion to improve the traceability of the new TODO comment.