Generate and use C.UTF-8 by default in mkarchroot
systemd >= 251 is forcing nspawns to use LANG=C.UTF-8: https://github.com/systemd/systemd/commit/b626f6959bcee11d966f96bd29a00502f4aa2ce4 If we are not going to enable glibc to generate this locale by default, we may want to generate it here.
(Not applicable if it's fixed in systemd or glibc instead: https://bugs.archlinux.org/task/74864)