.gitlab-ci.yml: Install Rust and cargo

.gitlab-ci.yml: Install Rust and cargo

Unfortunately, Rust 1.41 (as available in Debian 10) is too old for
object v0.23.0, a dependency of maturin:

error[E0658]: subslice patterns are unstable
   --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/object-0.23.0/src/read/mod.rs:162:41
    |
162 |             [0x7f, b'E', b'L', b'F', 1, ..] => FileKind::Elf32,
    |                                         ^^
    |
    = note: for more information, see https://github.com/rust-lang/rust/issues/62254

... so we have to resort to rustup.