Fix CI: macOS Python toolcache permissions, Windows pip invocation

- Create /Users/runner directory on macOS before setup-python (permission fix)
- Use `python -m pip` everywhere instead of calling pip directly (Windows fix)
- Refactor build_sidecar.py to use pip_install() helper via python -m pip

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -32,13 +32,17 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Create Python toolcache directory (macOS)
+        if: matrix.platform == 'macos'
+        run: sudo mkdir -p /Users/runner && sudo chown $USER /Users/runner
+
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
           python-version: ${{ env.PYTHON_VERSION }}
 
       - name: Install Python build tools
-        run: pip install --upgrade pip setuptools wheel
+        run: python -m pip install --upgrade pip setuptools wheel
 
       - name: Build sidecar
         working-directory: python