rust: ci: add job that runs Rust tools

rust: ci: add job that runs Rust tools

Code checks, as well as documentation generation, are not yet tied
to "make check" because they need new version of the Rust toolchain
(even nightly in the case of "rustfmt").  Run them in CI using the
existing nightly-Rust container.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>