check_export.yml: set a default reflex_version in code

this affects non workflow_dispatch triggers

.github/workflows/check_export.yml

@@ -57,8 +57,7 @@ jobs:
           python -m venv venv
           source venv/bin/activate
 
-          # pin reflex version - no moving target
-          pip install 'reflex${{ github.event.inputs.reflex_version }}'
+          pip install 'reflex${{ github.event.inputs.reflex_version || '==0.2.7' }}'
           pip install -r requirements.txt
           reflex init
           reflex export