[WIP] Downgrade rand to 0.9

[frozenspider]

Fixes #16
Revert rand to 0.9, and force-restrict crypto-bigint to be below 0.7.0-rc.10 since it depends on rand 0.10

[frozenspider]

This issue also affects cargo llvm-cov command, it tries to compile rand_core v0.10.0-rc-3 that depends on chacha20 (as opposed to rand_chacha), but it's unclear where is this dependency coming from. Seems to be architecture-specific, as I can't reproduce it on my aarch64 Debian, but GitHub Action does hit it.
After spending a few hours to try to trace it to no avail, I think it would be better to disable the code coverage action altogether for now, until rand fixes their crap.