Conda download caching keyed artifacts on channel, name, version and
filename only, but the same filename can hold different bytes in each
subdir (python_abi-3.11-4_cp311.conda in linux-64 and osx-arm64, for
example). A client on one platform could be served another platform's
package. The subdir is now part of the cached filename, so each one gets
its own entry. Entries cached under the old key are no longer used and
are re-fetched on first request.
