Times out returning very large artifact

Attempting to start a build of nvidia-cuda-toolkit (currently ~6GiB) causes the Debusine server to hang (#457 (closed)) forever, and eventually timeout with a 504. e.g.: https://debusine.debian.net/artifact/554528/download/?archive=tar.gz. That error-fails the task (e.g. https://debusine.debian.net/work-request/12028/).

This is because the tar.gz endpoint works entirely in-memory due to a Django bug.

We could work-around that Django bug with some monkey-patching etc. But... for any artifact of non-negligible size downloading the files separately is probably more efficient for client and server (not to mention, retryable). How about we:

1. Implement a archive-contents size limit for the tar.gz download endpoint (of, say, 100MiB)
2. Have the debusine client download artifact files separately if any of them are bigger than 20MiB or the total is bigger than the size limit above.