Fix JoinHandle import issue by enabling 'rt' feature in Tokio

[tahsintunan]

Description

This PR resolves the build failure in zksync_prover_job_processor caused by the following error:

error[E0432]: unresolved import tokio::task::JoinHandle
   --> crates/lib/prover_job_processor/src/job_runner.rs:1:5
    |
1   | use tokio::task::JoinHandle;
    |     ^^^^^^^^^^^^^^^^^^^^^^^ no JoinHandle in task

The error occurred because JoinHandle was gated behind the rt feature in Tokio, which was disabled. Enabling the feature resolves the import and fixes the build.