fix: restore UV_SYSTEM_PYTHON env var in TCK workflow

The uv pip install command fails without a virtual environment unless
UV_SYSTEM_PYTHON=1 is set to allow system-wide installation.

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

Author: jmesnil

.github/workflows/run-tck.yml

@@ -13,6 +13,8 @@ on:
 env:
   # Tag/branch of the TCK
   TCK_VERSION: 1.0-dev
+  # Tells uv to not need a venv, and instead use system
+  UV_SYSTEM_PYTHON: 1
   SUT_URL: http://localhost:9999
 
 # Only run the latest job