Arg: no mdadm on Ubuntu 18.04.5 Desktop install disk?

[Ken D'Ambrosio]

>> It's not on the 20.04 live disk either as I just discovered. You can
>> install mdadm with the above command on the live ISO system, manually
>> assemble the disk array and proceed to the install. The installer
>> should see the logical volumes then.

Pretty sure it's never been on there -- at least, some five years ago, I 
was going down a similar road, and seem to recall bumping into the exact 
same issue, with the exact same solution: installing to the live boot 
environment for install (or, in my case, troubleshooting).

-Ken