get_image: Fix CI build URLs

The URL for the test and CI image are not built in the same way, coming from separate branches. This fixes it until it get sorted out in image-builder.
2 jobs for master in 2 minutes and 12 seconds (queued for 28 seconds)
Status Name Job ID Coverage
  Build
passed build:deb #6277
librem5

00:01:42

passed build:docs #6276
librem5

00:02:12