Remove unnecessary env variable

On Windows, the action is failing on line:

    cd $GITHUB_WORKSPACE/python

The `$GITHUB_WORKSPACE` variable should be unnecessary here anyway.

Author: nickspoons

.github/workflows/tox.yml

@@ -30,5 +30,5 @@ jobs:
 
     - name: Run Test
       run: |
-        cd $GITHUB_WORKSPACE/python
+        cd python
         tox -e py