fix: add --system flag to PyYAML installation in CI

- Fix 'No virtual environment found' error in validate-workflows job
- Use uv pip install --system PyYAML for CI environment

Author: fgogolli

.github/workflows/ci.yml

@@ -99,7 +99,7 @@ jobs:
         cache-dependency-glob: "pyproject.toml.template"
 
     - name: Install PyYAML
-      run: uv pip install PyYAML
+      run: uv pip install --system PyYAML
 
     - name: Validate workflows
       run: make validate-all