Fix cefpython3 not importable during unit tests

_test_runner.py os.chdir()s to unittests/, so the repo root (where
cefpython3/ lives) falls off sys.path. Set PYTHONPATH to the workspace
root so the package is findable without installing it.

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

Author: linesight

.github/workflows/ci-windows.yml

@@ -129,6 +129,8 @@ jobs:
 
       - name: Run unit tests
         run: python unittests/_test_runner.py
+        env:
+          PYTHONPATH: ${{ github.workspace }}
 
   wheel:
     needs: test