fix: use consistent uv pattern for PyYAML installation

- Follow same pattern as other CI jobs: uv venv then uv pip install
- Maintain consistency with build dependencies installation pattern
- Remove --system flag in favor of virtual environment approach

Author: fgogolli

.github/workflows/ci.yml

@@ -99,7 +99,9 @@ jobs:
         cache-dependency-glob: "pyproject.toml.template"
 
     - name: Install PyYAML
-      run: uv pip install --system PyYAML
+      run: |
+        uv venv
+        uv pip install PyYAML
 
     - name: Validate workflows
       run: make validate-all