fix: update rand to 0.9.3 for dependabot alert #8 (#336)

Dependabot alert `#8` tracks `GHSA-cq8v-f236-94qc` for `rand`.

In this workspace, the runtime `rand 0.9.x` path comes from `secp256k1`
and `twox-hash`, and both accept the patched `0.9.3` release. This
change updates that vulnerable runtime path with the smallest possible
dependency change.

[The
alert](https://github.com/NomicFoundation/solx/security/dependabot/8) is
low-priority and not critical for this repo. Can be merged after the
cargo cooldown.

Author: hedgar2017