ci: use vars for BROWSERSTACK_USERNAME

Author: fadi-george

.github/workflows/appium-e2e.yml

@@ -77,7 +77,7 @@ jobs:
       - name: Run Appium tests
         working-directory: _sdk-shared/appium
         env:
-          BROWSERSTACK_USERNAME: ${{ secrets.BROWSERSTACK_USERNAME }}
+          BROWSERSTACK_USERNAME: ${{ vars.BROWSERSTACK_USERNAME }}
           BROWSERSTACK_ACCESS_KEY: ${{ secrets.BROWSERSTACK_ACCESS_KEY }}
           BROWSERSTACK_APP_URL: ${{ steps.bs-upload.outputs.app-url }}
           SDK_TYPE: ${{ inputs.sdk-type }}