Re-enable beta Rust tests in CI

[teor2345]

Motivation

We want to test Zebra with the next Rust release candidate, so we can report any compilation bugs and get them fixed.

On 29 July, Zebra's beta Rust tests started failing, due to a beta rust bug (rust-lang/rust#87483).

But the stable CI tests and beta zebra-chain build were fine.

Actions

We disabled the beta Rust tests in PR #2542.

We should re-enable beta Rust:

• after the fix is backported to beta
  • Mir borrowck does not generate lifetime variables for 'static lifetimes during opaque type resolution rust-lang/rust#87483
  • Backport PR 87483 rust-lang/rust#88190
• do we need to wait for a beta point release, or are we using daily beta builds?

[teor2345]

I removed docs from this ticket, because they aren't impacted by the beta rust bug.

[teor2345]

I confirmed that this Rust PR fixes the test build failure we're seeing: rust-lang/rust#87483

So we should be good to re-enable beta once it has been backported.