ci: install uv before running subprocess lint in check-ubuntu

The `Lint subprocess spawns` step calls `uv run python
ci/find_direct_subprocess.py --fail` but the job never installed
`uv`: `zackees/setup-soldr@v0` only installs Rust tooling. Add the
same `astral-sh/setup-uv@v3` step the dedicated `lint-subprocess.yml`
workflow already uses so this step can actually execute.

Addresses CodeRabbit review on #200.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zackees

.github/workflows/check-ubuntu.yml

@@ -29,6 +29,7 @@ jobs:
         run: soldr cargo check --workspace --all-targets
       - name: Clippy
         run: soldr cargo clippy --workspace --all-targets -- -D warnings
+      - uses: astral-sh/setup-uv@v3
       - name: Lint subprocess spawns
         run: uv run python ci/find_direct_subprocess.py --fail
       - name: Test