CI: Check that the lockfile is up to date.

For at least one job, run the first cargo run with --locked.  This
will fail if the lockfile needs updating.

I have verified that this correctly detects this situation:
  https://gitlab.torproject.org/Diziet/arti/-/pipelines/37692
failed.  Now I have rebased this branch onto main to get the fix to
Cargo.lock.

.gitlab-ci.yml

@@ -25,7 +25,7 @@ rust-latest:
   image: rust:latest
   script:
     - rustup show
-    - cargo build --verbose --target x86_64-unknown-linux-gnu
+    - cargo build --locked --verbose --target x86_64-unknown-linux-gnu
     - cargo test --verbose --target x86_64-unknown-linux-gnu
     - rustup component add clippy rustfmt
     - rustup show