fix(nix): initialize hashes.json with per-system format

[jerome-benoit]

Fixes a bug in the update-hashes.sh script introduced in PR #8033.

Problem

The script was initializing hashes.json with legacy string format:

{"nodeModules": "sha256-AAA..."}

But the write_node_modules_hash function expects per-system object format:

{"nodeModules": {}}

This caused the update-nix-hashes workflow to fail when trying to write system-specific hashes.

Solution

1. Initialize with empty object {} instead of dummy string
2. Fix validation check to match per-system format

Changes

• nix/scripts/update-hashes.sh line 13: Change init from string to object
• nix/scripts/update-hashes.sh line 114: Update validation to check per-system hash

Related: #8029, #8033

[github-actions]

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2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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[Copilot]

Pull request overview

This PR fixes a bug in the update-hashes.sh script where the initialization of hashes.json was using a legacy string format instead of the per-system object format, causing the update-nix-hashes workflow to fail.

Changes:

• Initialize nodeModules with empty object {} instead of dummy string value
• Update validation check to verify per-system hash format using $system key

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[jerome-benoit]

@rekram1-node: follow up fix for #8029, #8033