Fix failing "Test Download" scheduled Pipeline job

After my last CI fix, one of the daily scheduled Test Download jobs started to fail as it implicitly relied on the old cabal behaviour which was changed in the new version. This tries to fix that by switching these jobs back to the old image. This also explicitly sets the main image rather than using latest, this should make changing the image in the future easier, as one can create and push the new image without breaking the existing setup and then make a commit to change the selected image, taking advantage of CI to test image change explicitly.