CI failed by unsafe repository

[buzztaiki]

e.g. https://github.com/ruby/rbs/runs/6148027791?check_suite_focus=true#step:9:7

According to the following, set-safe-directory has been added to the checkout action to address this issue.

• fatal: unsafe repository (REPO is owned by someone else) with ubuntu 20.04 container actions/checkout#760
• Add set-safe-directory input to allow customer to take control. actions/checkout#770