Use smaller images in CI for downloading data

[mattcieslak]

I bet we could shave off 3 minutes from the CI run if we switch to a tiny image for downloading the test data.

[tsalo]

I had a similar hope for XCP-D in PennLINC/xcp_d#636. I ended up giving up on it, but if we get it working in QSIRecon I'd love to copy the changes over to our other packages.