Fix the rust version used in pipeline

[limemloh]

Purpose

Fix issue causing the rust-bindings wasm module to throw runtime error: unreachable.

Changes

• Use a fixed Rust toolchain version everywhere in the pipeline. This will fix an issue in the deployment, since the wasm-pack version 0.12.1 seems incompatible with the rust version actually used by the CI (which was 1.79 by accident).
• Add unit test catching the above issue, when we update the rust version sometime in the future.
• Correct path for caching of concordium-contracts-common workspace.
• Remove unused type AttributeRandomness.

[soerenbf]

Looks good 👍 😄