Lubuntu groovy (daily) won't boot - "isolinux.bin missing or corrupt." on older BIOS boxes

[Chris Guiver]

Lubuntu groovy daily QA-test (2020-09-11)

The ISO has been written twice to thumb-drive, it'll boot on UEFI boxes, but fails to boot on

hp dc7700 (c2d-e6320, 5gb, nvidia quadro nvs 290)
hp dc7900 (c2d-e8400, 4gb, intel 4 series integrated i915)
dell [optiplex] 755 (c2d-e6850, 5gb, amd/ati radeon rv516/x1300/x1550)
dell [optiplex] 755 (c2d-e8300, 8gb, amd/ati radeon rv610/radeon hd2400 pro/xt)
dell [optiplex] 780 (c2q-q9400, 8gb, amd/ati cedar radeon hd 5000/6000/7350/8350)

error being reported as

"isolinux.bin missing or corrupt"

also fails on

dell [optiplex] 990 (i7-2600, 16gb, nvidia geforce gt 6600 gt)

but it reports via what I believe is a BIOS/UEFI message that the "selected boot device failed \n press any key to restart" (if other messages were visible they weren't readable disappearing too quickly; the screen blanking when it displays the message I quote)

(though some boxes immediately so message is flashed & unreadable, and box moves to next boot device which is usually HDD & it boots a prior QA-test install)

This maybe related to https://bugs.launchpad.net/ubuntu-cdimage/+bug/1886148

This impacts 20200911.1 and Xubuntu 20200912 too