install_arch: Use shell instead of command

shell is like command but runs the command through a shell, so that
piping will work.

roles/install_arch/tasks/main.yml

@@ -165,7 +165,7 @@
   template: src=sshd_config.j2 dest=/mnt/etc/ssh/sshd_config owner=root group=root mode=0644
 
 - name: clean pacman cache
-  command: yes | chroot /mnt pacman -Scc
+  shell: set -o pipefail && yes | chroot /mnt pacman -Scc
   register: chroot_pacman_clean_cache
   changed_when: "chroot_pacman_clean_cache.rc == 0"