cp: fix flaky test test_cp_arg_interactive_update, document adjacent bug#6020

cakebaker merged 1 commit intouutils:mainuutils/coreutils:mainfrom

BenWiederhake:dev-cp-flaky-arg_interactive_updateBenWiederhake/coreutils-rs:dev-cp-flaky-arg_interactive_updateCopy head branch name to clipboard