Combine OVA generation steps

Previously this was done in three derivations (one to build the raw
disk image, one to convert to OVA, one to add a hydra-build-products
file). Now it's done in one step to reduce the amount of copying
to/from S3. In particular, not uploading the raw disk image prevents
us from hitting hydra-queue-runner's size limit of 2 GiB.