chore: only run GitHub actions when on main account (#4433)
the most prominent case being update-copyright-years-in-license-file altering commit history in forks depending on configuration (sometimes these type of PRs just auto-merge on forks, creating a merge conflict when they sync their fork for preparing a PR)
additionally, some actions require secrets so they would fail anyway
Co-authored-by: Daniel Gibbs <me@danielgibbs.co.uk>