Support automatic baseline downloads for jit-diff

https://github.com/dotnet/runtime/pull/42511 introduced automatic baseline downloads for JITs from the JIT rolling build introduced in https://github.com/dotnet/runtime/pull/42507, for SuperPMI asm diffs run from superpmi.py.

This same automatic download functionality should be added to (non-SuperPMI) jit-diff.

They should share the same baseline JIT cache.
