core: chroot: Deduplicate supported arch list (MR 2476)

self.arch is a property and calls Arch.from_str, which errors upon
encountering an unknown architecture. Therefore, the error message is
changed and needs to be adjusted in the tests. Since Arch.supported is a
set, error messages were not deterministic before, so we need to sort
the list of architectures now.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>