Reconsider cache download buffer size

[stucka]

cache.download now grabs 8k at a time; 1024*1024, a megabyte, may be more performant.

[newsroomdev]

Not a bad idea. San Diego is running cache.download in roughly 5mins. We can revisit later once we're given the all-clear that we can run additional scrapers in production. I am grabbing a megabyte at a time in clean-prefect/flows/assets.py's streaming download, so parity would be nice if we need to debug