Fix: Remove activate-environment parameter

The setup-lecture-env action hardcodes 'quantecon' as the environment name,
so this parameter is not needed and was causing a warning.

Author: mmcky

.github/workflows/cache.yml

@@ -17,7 +17,6 @@ jobs:
         with:
           python-version: '3.13'
           environment-file: 'environment.yml'
-          activate-environment: 'quantecon'
 
       - name: graphviz Support # TODO: required?
         run: |