Skip nixfmt hook on Windows

[adamtheturtle]

nixfmt depends on the Haskell unix package, which is not buildable on Windows.

This fixes the lint failure on Windows seen in #1923 and other PRs.

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Note

Low Risk
Workflow-only change that just adjusts which pre-commit hooks run on Windows; no production code or data paths are affected.

Overview
Updates the GitHub Actions lint workflow to also skip the nixfmt pre-commit hook on Windows (in addition to hadolint), avoiding Windows CI failures due to an unbuildable Haskell dependency.

^{Written by Cursor Bugbot for commit 7f38ba0. This will update automatically on new commits. Configure here.}