Move build-and-test onto a public runner

[tothtamas28]

The build-and-test job ran on a self-hosted runner inside a custom Docker container. This PR moves it to ubuntu-latest, dropping the Docker setup entirely.

Additionally, all jobs now rely on rust-toolchain.toml for the Rust version: build-and-test picks it up automatically via rustup, and build-ffi-and-fuzz (via with-docker) reads it at build time and passes it to docker build.

Changes:

• Add rust-toolchain.toml, pinning the Rust version
• Remove Dockerfile and the Docker setup from build-and-test; run directly on ubuntu-latest
• Read the Rust version from rust-toolchain.toml in with-docker and pass it to docker build