ci (yamltests): set PYTHONPATH

util functions need the search path to include current directory to
find python files under custom_components/tuya_local

.github/workflows/yamltests.yml

@@ -22,6 +22,9 @@ jobs:
       matrix:
         python-version: ['3.13']
 
+    env:
+      PYTHONPATH: .
+
     steps:
       - uses: actions/checkout@v4
         with: