Use C.UTF-8 as the default locale

The glibc 2.35-6 package ships with the C.UTF-8 locale included which
means there is now a usable UTF-8 locale available by default.

en_US.UTF-8 will still be generated because PostgreSQL clusters are
created with that locale. Migrating the clusters to C.UTF-8 is
possible, but that requires dumping and recreating them.

playbooks/tasks/sync-ssh-hostkeys.yml

@@ -20,7 +20,7 @@
         | sed 's#^127.0.0.1#{{ inventory_hostname }}#' \
         | sort
       environment:
-        LC_COLLATE: C  # to ensure reproducible ordering
+        LC_COLLATE: C.UTF-8  # to ensure reproducible ordering
       args:
         executable: /bin/bash
       register: known_hosts

roles/common/templates/locale.conf.j2

-LANG=en_US.UTF-8
-LC_COLLATE=en_US.UTF-8
-LC_TIME=en_US.UTF-8
+LANG=C.UTF-8

roles/install_arch/tasks/main.yml

@@ -112,7 +112,7 @@
   changed_when: "chroot_locale_gen.rc == 0"
 
 - name: run systemd-firstboot
-  command: chroot /mnt systemd-firstboot --locale=en_US.UTF-8 --timezone=UTC --hostname={{ hostname }}
+  command: chroot /mnt systemd-firstboot --locale=C.UTF-8 --timezone=UTC --hostname={{ hostname }}
   register: chroot_systemd_firstboot
   changed_when: "chroot_systemd_firstboot.rc == 0"