Catalyst is still fighting me. This morning I downloaded an official stage 3 tarball for `aarch64`, set up Catalyst in that, and tried again. Same result.
While a few things have changed since the days when I did this for the Gentoo/MIPS team, it's not _that_ different today. Either way, I can't see how Python's `TemporaryDirectory()` is going to successfully remove a directory that is being used as a mountpoint.
https://bugs.gentoo.org/971617
