Fix server Docker builds for current Rust toolchain and CUDA feature wiring

[Copilot]

The build-docker Actions job was failing in the server image build path due to an outdated Rust/Cargo toolchain in the Docker builders. The CUDA image path also referenced a package feature that was never declared at the crate level.

• Align Docker builders with CI Rust version

  • Update the CPU builder image from Rust 1.75 to Rust 1.87.
  • Install Rust 1.87 explicitly in the CUDA builder instead of relying on Ubuntu-packaged rustc/cargo.

• Force llama.cpp builds to use Clang in containers

  • Set CC=clang and CXX=clang++ in both server Docker builders so the native llama-cpp build uses the expected compiler toolchain.

• Declare the crate-level CUDA feature

  • Add a top-level cuda feature in mohawk-server/Cargo.toml and forward it to llama-cpp-2/cuda, matching the existing Dockerfile.cuda build invocation.

• Net effect

  • The CPU Docker build now uses a toolchain compatible with current crate resolution.
  • The CUDA Docker build no longer fails immediately on an undefined Cargo feature.

[features]
cuda = ["llama-cpp-2/cuda"]