Add full Rust CI to gitlab-ci

The Rust plugin is the first part of nbdkit that had any CI coverage, added by https://github.com/libguestfs/nbdkit/pull/8 . Gitlab CI was added later, and never included comprehensive CI for the Rust plugin; just a simple build check. But ever since the github mirror was disconnected, Cirrus hasn't been running. Since Cirrus-CI does not support Gitlab, we need to merge all of the logic from Cirrus to Gitlab-CI.