ci: Remove invalid "required" keyword associated with actions/download-artifact

This commit fixes the following error introduced by adb85d69c (ci: Make
cache download optional):

  Unexpected input(s) 'required', valid inputs are ['name', 'path']

.github/workflows/main.yml

@@ -891,7 +891,6 @@ jobs:
         with:
           name: cache
           path: ./cache
-          required: false
 
       - name: load base
         run: xz -d -k < ./cache/base.tar.xz | docker load