fix(ci): replace Blacksmith runner in generate workflow

[amdmax]

Summary

• Replace hardcoded blacksmith-4vcpu-ubuntu-2404 runner in generate.yml with ${{ vars.RUNNER_UBUNTU || 'blacksmith-4vcpu-ubuntu-2404' }}}
• Follows the same pattern already applied to test.yml, typecheck.yml, deploy.yml, and nix-hashes.yml
• Prevents workflow failures on forks without Blacksmith runners

Test plan

• Verify generate workflow runs successfully without runner label errors

🤖 Generated with Claude Code

[github-actions]

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