fix(release_util): Fix DIGESTS regression from feb59db9f

While attempting to fix the easy to mix up DIGESTS names in feb59db9f I
stumbled across yet another way that the DIGESTS names were a bit
unpredictable: previously a .bz2 got stuck into the file name when
upload_images automatically compressed some file types. The new code
missed this and never added the .bz2. Correct this now, both for
image_to_vm which has a pile of glue to keep the legacy names in place
for now and build_image which I never intended to change.
Merge pull request #180 from marineam/fix-digests-again

fix(release_util): Fix DIGESTS regression from feb59db9f