modules: add edgeCacheSubstituters options

[colemickens]

Summary by CodeRabbit

• New Features
  • Added configuration options to specify edge cache substituter URLs for Determinate Nixd in NixOS and nix-darwin environments.

[lucperkins]

LGTM

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: de02b2b2-3376-41c7-8b6d-3e65fd9615fd

📥 Commits

Reviewing files that changed from the base of the PR and between efa5aa3 and 614ed7e.

📒 Files selected for processing (2)

• modules/nix-darwin/default.nix
• modules/nixos.nix

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📝 Walkthrough

Walkthrough

This PR adds edgeCacheSubstituters configuration option to both nix-darwin and NixOS modules. When set, the option specifies a list of substituter URLs that are serialized into the generated determinate/config.json file on each platform.

Changes

Edge cache substituters across platforms

Layer / File(s)	Summary
nix-darwin edgeCacheSubstituters support modules/nix-darwin/default.nix	Adds determinateNix.determinateNixd.edgeCacheSubstituters option as a nullable list of strings, and extends the JSON generation allowlist to conditionally emit the edgeCacheSubstituters attribute to config.json when set.
NixOS edgeCacheSubstituters support modules/nixos.nix	Adds determinate.edgeCacheSubstituters option as a nullable list of strings, and conditionally generates /etc/determinate/config.json with the edgeCacheSubstituters key when the option is non-null.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A config so neat, with cache URLs bright,
On Darwin and NixOS, spanning both sites,
Substituters declared, in JSON they rest,
Two platforms aligned, both passing the test!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely describes the main change: adding new edgeCacheSubstituters options to both the nix-darwin and NixOS modules.
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

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch colemickens/fec

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