Debian/Ubuntu: Set hostname in Live environment

This ensures that the Live hostname does not leak into e.g. OpenSSH
server keys.  It's not a huge deal either way, but this is nicer.

Closes #158
Thanks: anarcat
Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS for Raspberry Pi.rst

@@ -561,7 +561,8 @@ Step 4: System Configuration
 
    Replace ``HOSTNAME`` with the desired hostname::
 
-     echo HOSTNAME > /mnt/etc/hostname
+     hostname HOSTNAME
+     hostname > /mnt/etc/hostname
      vi /mnt/etc/hosts
 
    .. code-block:: text

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -672,7 +672,8 @@ Step 4: System Configuration
 
    Replace ``HOSTNAME`` with the desired hostname::
 
-     echo HOSTNAME > /mnt/etc/hostname
+     hostname HOSTNAME
+     hostname > /mnt/etc/hostname
      vi /mnt/etc/hosts
 
    .. code-block:: text