gitlab-ci: Test librem5-devkit-flash-image download

We can't test the actual flashing but that's most of what the script
does atm.

This will make us spot things like fixed in !1 early.
3 jobs for download-only in 3 minutes and 7 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build:deb #8271
librem5

00:01:28

passed build:docs #8270
librem5

00:01:51

 
  Test
passed test:download #8272
librem5

00:01:15