[nix] Extract Go derivation to nested flake for cross-repo reuse

[maru-ava]

Why this should be merged

Exposing the go derivation via a flake allows the firewood flake to use it to provide its go version without preventing the avalanchego flake from depending on the firewood flake.

How this was tested

• CI
• Manually
  • Locally: nix ./nix/go
  • Remotely: nix run 'github:ava-labs/avalanchego?dir=nix/go&ref=9cd1c3aee7c2ef1b46621799d7ef0698c2326046'

Need to be documented in RELEASES.md?

N/A

[Copilot]

Pull Request Overview

Refactors Go derivation into a nested flake structure to enable cross-repository reuse while maintaining dependency relationships between avalanchego and firewood projects.

• Extracts Go toolchain definition from nix/go.nix into a standalone flake (nix/go/flake.nix)
• Updates main flake to consume Go package from nested flake instead of direct import
• Updates documentation reference to reflect new file location

Reviewed Changes

Copilot reviewed 3 out of 6 changed files in this pull request and generated no comments.

File	Description
nix/go/flake.nix	Creates new standalone flake exporting Go toolchain package
flake.nix	Updates to use go-flake input instead of direct import
go.mod	Updates comment to reference new Go derivation file location

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[Elvis339]

Just to confirm I understand the issue correctly; before this change, did the firewood flake need to depend on the entire avalanchego flake just to get the Go version, which would create a circular dependency since avalanchego also depends on firewood? And this PR solves it by letting firewood import only the Go derivation from nix/go instead of the full avalanchego flake - is that right?

[maru-ava]

Just to confirm I understand the issue correctly; before this change, did the firewood flake need to depend on the entire avalanchego flake just to get the Go version, which would create a circular dependency since avalanchego also depends on firewood? And this PR solves it by letting firewood import only the Go derivation from nix/go instead of the full avalanchego flake - is that right?

Correct. And I'd like to reuse avalancheg's go derivation in the firewood flake instead of:

1. Using the nix version which is not guaranteed to match the version defined in firewood's go.mod
   • This will still work because golang is smart enough to download and switch to the correct version, but cleaner to just use the correct version to start with
2. Duplicating the go derivation in the firewood repo