ci(check-ubuntu): install uv so subprocess lint step can run

[zackees]

Summary

• The Lint subprocess spawns step added to check-ubuntu.yml in Migrate subprocess spawns to running-process + regression linter #200 invokes uv run python ci/find_direct_subprocess.py --fail, but the job never installs uv. Main broke on the merge of Migrate subprocess spawns to running-process + regression linter #200 with uv: command not found (exit 127).
• Fix: add - uses: astral-sh/setup-uv@v3 before the step, matching how the sibling lint-subprocess.yml workflow sets up uv.

Failing run: https://github.com/FastLED/fbuild/actions/runs/24905357266

Test plan

• CI Check (ubuntu-latest) passes on this PR (previously failed at the lint step).
• Lint subprocess spawns workflow remains green (unchanged).

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

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Run ID: 91735d8d-bf19-4c5b-98a7-6bdac9f346db

📥 Commits

Reviewing files that changed from the base of the PR and between d088072 and 3f54689.

📒 Files selected for processing (1)

• .github/workflows/check-ubuntu.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/check-ubuntu-install-uv

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