mkarchroot: generate and use C.UTF-8 by default

systemd >= 251 is forcing nspawns to use LANG=C.UTF-8:
systemd/systemd@b626f69

It makes sense to generate the C.UTF-8 locate here when using it as the
default LANG value.

Related to: https://bugs.archlinux.org/task/74864

Author: felixonmars

mkarchroot.in

@@ -84,8 +84,8 @@ done
 unshare --mount pacstrap -${umode}Mcd ${pac_conf:+-C "$pac_conf"} "$working_dir" \
 	"${cache_dirs[@]/#/--cachedir=}" "$@" || die 'Failed to install all packages'
 
-printf '%s.UTF-8 UTF-8\n' en_US de_DE > "$working_dir/etc/locale.gen"
-echo 'LANG=en_US.UTF-8' > "$working_dir/etc/locale.conf"
+printf '%s.UTF-8 UTF-8\n' C en_US de_DE > "$working_dir/etc/locale.gen"
+echo 'LANG=C.UTF-8' > "$working_dir/etc/locale.conf"
 echo "$CHROOT_VERSION" > "$working_dir/.arch-chroot"
 
 systemd-machine-id-setup --root="$working_dir"