README: Improve cluster formation docs

- Begin code block with sudo to remind the following shall be ran as root.
- Remove hardcoded key, instead copy from ubutnu user.

README.md

@@ -161,6 +161,8 @@ Added some ARM64 workers by using Ubuntu 22.04 server on Raspberry Pi.
 After machines have booted up and you can reach them via SSH:
 
 ```bash
+sudo -i
+
 # Enable required kernel modules
 cat > /etc/modules << EOF
 overlay
@@ -200,10 +202,8 @@ systemctl disable --now multipathd snapd bluetooth ModemManager hciuart wpa_supp
 # Permit root login
 sed -i -e 's/PermitRootLogin no/PermitRootLogin without-password/' /etc/ssh/sshd_config
 systemctl reload ssh
-cat << EOF > /root/.ssh/authorized_keys
-sk-ecdsa-sha2-nistp256@openssh.com AAAAInNrLWVjZHNhLXNoYTItbmlzdHAyNTZAb3BlbnNzaC5jb20AAAAIbmlzdHAyNTYAAABBBD4/e9SWYWYoNZMkkF+NirhbmHuUgjoCap42kAq0pLIXFwIqgVTCre03VPoChIwBClc8RspLKqr5W3j0fG8QwnQAAAAEc3NoOg== lauri@lauri-x13
-EOF
-userdel -f ubuntu
+cat ~ubuntu/.ssh/authorized_keys > /root/.ssh/authorized_keys
+userdel -fr ubuntu
 apt-get install -yqq linux-image-generic
 apt-get remove -yq cloud-init linux-image-*-kvm
 ```