rust-cache doesn't seem to work on non-linux hosts

[2opremio]

See all the Build jobs at: https://github.com/stellar/soroban-rpc/actions/runs/9584445833/job/26428082692

The make build-preflight step (which builds the libpreflight Rust library) is really fast on Linux (both amd64 and arm64) where it takes less than 30 seconds.

On MacOS and Windows it takes around 2 minutes, which makes me think that caching is not working.

Tentatively marking as a bug.