fix: don't modify ssh_config for ubuntu-cpc projects (LP:1968873)

The cloud-init bug (see LP:1968873) got fixed now so using a sshd
config snippet should work now.

This partly reverts commit aa1be5eaaa
but uses now 60-cloudimg-settings.conf instead of
10-cloudimg-settings.conf .

live-build/ubuntu-cpc/hooks.d/chroot/052-ssh_authentication.chroot

@@ -1,3 +1,8 @@
 #!/bin/bash
 
-sed -i "s|#PasswordAuthentication yes|PasswordAuthentication no|g" /etc/ssh/sshd_config
+# NOTE: the file number needs to be **higher** than the one
+# cloud-init writes (cloud-init uses 50-cloud-init.conf) so
+# the cloud-init file takes precedence
+cat << EOF >> "/etc/ssh/sshd_config.d/60-cloudimg-settings.conf"
+PasswordAuthentication no
+EOF