ubiquity grub-install fails with /boot on RAID, ubiquity does not install mdadm, workaround included

[Jim At Your Service]

With Ubuntu 18.04

Ubiquity installer should install mdadm before running grub-install if /boot is on a RAID disk.

There have been multiple bug reports filed over the years against grub-installer.
The grub-installer is fine. To support /boot on RAID, mdadm should be pre-installed.

Various recipes explain to work-around the issue just drop to a shell after the installer complains, and chroot to the target, and install mdadm, then do grub-install. Not so easy.

In my case, I have a ubiquty/success_command that has to run, and the workaround requires
allowing ubiquity to fail, and so the success_command never happens, nor does any final cleanup.

The workaround I used was to to hack on the live-CD, which must be done after boot,
or in my case remains persistent because I made a persistent live-CD using mkusb.

vi /usr/share/grub-installer/grub-installer
        # near line 647 change from
        apt-install $grub_package || exit_code=$?
        # change to
        # 13.0.6.17.15-t10 jimays Hack to make sure mdadm is installed so grub-install will succeed with RAID.
        apt-install -y mdadm $grub_package || exit_code=$?

And then ubiquity succeeds and grub installs with /boot on /dev/md1.
I am not sure where the actual proper place to install mdadm is
or what the proper test would be to see if it is actually needed.