Fix CI: add --system flag to uv pip install

In GitHub Actions environment, uv requires --system flag when
no virtual environment is present.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: alingse

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
           echo "$HOME/.local/bin" >> $GITHUB_PATH
       - name: Install dependencies
         run: |
-          uv pip install -e ".[dev]"
+          uv pip install --system -e ".[dev]"
       - name: Run ruff
         run: ruff check .
       - name: Run mypy
@@ -45,7 +45,7 @@ jobs:
           echo "$HOME/.local/bin" >> $GITHUB_PATH
       - name: Install dependencies
         run: |
-          uv pip install -e ".[dev]"
+          uv pip install --system -e ".[dev]"
       - name: Run tests
         run: pytest --cov=jsoncsv --cov-report=xml
       - name: Upload coverage
@@ -68,8 +68,8 @@ jobs:
           echo "$HOME/.local/bin" >> $GITHUB_PATH
       - name: Build package
         run: |
-          uv pip install build
-          uv pip install twine
+          uv pip install --system build
+          uv pip install --system twine
           python -m build
       - name: Check package
         run: twine check dist/*