nix: declare the Cachix cache in the flake and document the tagged-release caveat

[kixelated]

Summary

Makes nix run github:moq-dev/moq#moq-relay actually benefit from our Cachix cache, and documents the one caveat that previously bit consumers silently: only tagged releases are cached.

• flake.nix: added a nixConfig block declaring kixelated.cachix.org as an extra-substituters + extra-trusted-public-keys. With it, nix run ... --accept-flake-config consults the binary cache automatically, no manual nix.conf edit or prior cachix use required. The old comment is replaced with one that spells out the tagged-releases-only behavior and the pin-to-a-tag pattern.
• doc/setup/linux.md: the Nix bullet previously just said "use the binary cache to skip building from source." It now notes that only tagged releases are pushed, shows the pinned --accept-flake-config command, and warns that the unpinned form tracks the default branch and compiles from source.

Why

cachix.yml only fires on moq-relay-v* / moq-cli-v* / sibling tags and pins the last 10 revisions per package+OS. So a plain nix run github:moq-dev/moq#moq-relay resolves to the default-branch HEAD, which is never in the cache and falls back to building from source even when the substituter is configured. To get a cache hit you have to pin the flake ref to a recent tag:

nix run github:moq-dev/moq/moq-relay-v0.12.4#moq-relay --accept-flake-config

Reviewer notes

• --accept-flake-config is required because Nix won't silently apply substituters from an untrusted flake. cachix use kixelated (once) is the permanent alternative; both are documented.
• The examples hardcode v0.12.4 (current latest), which will go stale as new releases are tagged. Open to a follow-up that tracks "latest stable" instead of a pinned version if you'd prefer doc examples not need a bump each release.
• Docs/flake only, no wire or API change, so this targets main.

(Written by Claude)

[coderabbitai]

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Configuration used: Organization UI

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Run ID: d700eb6a-9bad-4dce-9da6-6d2c57270150

📥 Commits

Reviewing files that changed from the base of the PR and between f727748 and cbbe891.

📒 Files selected for processing (1)

• flake.nix

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Walkthrough

This PR enables Cachix-backed binary caching for the moq project's Nix flake. The flake.nix now declares a nixConfig block that registers the kixelated.cachix.org cache as a substituter with its signing key, eliminating the need for users to manually configure Cachix. The doc/setup/linux.md is updated to document a flake-based nix run workflow using a pinned version (moq-relay-v0.12.4) with --accept-flake-config to opt into this configuration, and explains that unpinned references track the default branch (not cached) while offering cachix use kixelated as a one-time alternative for permanent cache trust.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: declaring the Cachix cache in the flake and documenting the tagged-release limitation.
Description check	✅ Passed	The description is well-structured and comprehensively explains the purpose, implementation details, and rationale for the changes to both files.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch claude/vigilant-elbakyan-e417d9

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