do not add "+stable" option to cargo by default [Bug]

[milyin]

Describe the bug

@JEnoch wrote:
The problem comes for users that didn't install cargo via rustup, such as incoming Ubuntu Noble 24.04 where Rust 1.75 comes as a Debian package. In this case cargo doesn't understand at all the +<toolchain> option.

We see that using stable could lead to troubles if a new stable comes and break zenoh or zenoh-c build.
That's why we use rust-toolchain.tom in all our repo that fixes the Rust toolchain to a version we know it's building with. However, a user depending on zenoh uses the toolchain he wants...
BTW zenoh-c/rust-toolchain.toml is not respected using this +${ZENOHC_CARGO_CHANNEL}.

In my opinion, the default should be a empty or undefined ZENOHC_CARGO_CHANNEL that leads to no +<toolchain> option be used by default. Then in our CI, we shall set ZENOHC_CARGO_CHANNEL=1.75 (or any other version, but this one makes sure it works for ROS).

To reproduce

• Build zenoh-c on Ubuntu Noble 24.04 with Rust and cargo installed via sudo apt install cargo

System info

• Ubuntu Noble 24.04