librem5-flash-image: Allow to download luks images

Repurpose the board variant for that since it's unused.
3 jobs for pureos/byzantium in 3 minutes and 23 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #333236
librem5
build:deb

00:01:23

 
  Test
passed #333237
librem5
test:download:deb

00:01:45

passed #333238
librem5
test:download:pip

00:01:59