CI tests fail with Rust 1.70.0

[jpraynaud]

Issue

The Rust tests fail in the CI following the upgrade of Rust to 1.70.0. The -Z unstable-options has been deactivated with the non nightly builds of the toolchain

See https://github.com/input-output-hk/mithril/actions/runs/5157587712/jobs/9290193185#step:4:755:

     Running unittests src/lib.rs (target/debug/deps/mithril_aggregator-8a7a5c39a74437d2)
error: the option `Z` is only accepted on the nightly compiler
error: test failed, to rerun pass `-p mithril-aggregator --lib`
     Running unittests src/main.rs (target/debug/deps/mithril_aggregator-32710dd7c97845a4)
error: the option `Z` is only accepted on the nightly compiler