Add --system flag to uv pip install in Makefile

CI runners use system Python without a venv, so uv needs --system.

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

Author: MaxGhenis

Makefile

@@ -10,7 +10,7 @@ format:
 	ruff check .
 
 install:
-	uv pip install -e ".[dev]" --config-settings editable_mode=compat
+	uv pip install --system -e ".[dev]" --config-settings editable_mode=compat
 
 test-country-template:
 	policyengine-core test policyengine_core/country_template/tests -c policyengine_core.country_template