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

@@ -184,7 +184,7 @@
   file: src=/run/systemd/resolve/stub-resolv.conf dest=/mnt/etc/resolv.conf state=link force=yes owner=root group=root mode=0644
 
 - name: clean pacman cache
-  command: yes | chroot /mnt pacman -Scc
+  shell: yes | chroot /mnt pacman -Scc
   register: chroot_pacman_clean_cache
   changed_when: "chroot_pacman_clean_cache.rc == 0"