ci: add override flag to rust toolchain installation in release workflow

[0oM4R]

This PR updates the CI workflow to include the --override flag when installing Rust targets with rustup.

Details

Previously, the workflow used rustup target add without --override.

Added --override ensures the specified target is installed and available for the active toolchain in the workflow.
reference: https://github.com/actions-rs/toolchain?tab=readme-ov-file#inputs

This helps prevent build failures like:

error[E0463]: can't find crate for `core`
note: the `x86_64-unknown-linux-musl` target may not be installed

I tried the build on an example workflow on https://github.com/threefoldtech/rmb-rs/actions/runs/18095666645/job/51485995286
related issue:

• Release workflow regression on ubuntu-24.04 runner #235