Without a version the setup action installs the latest toolchain. Swift 6.4.0 shipped on 2026-09-15 and the swiftly build cached on the runner cannot install it, so the docs job started failing.
Without a version the setup action installs the latest toolchain. Swift 6.4.0 shipped on 2026-09-15 and the swiftly build cached on the runner cannot install it, so the docs job started failing.