[Page: Editor] [Rust] Exercise doesn't build (when external packages are required?)

This happened on the gigasecond exercise in rust:
https://exercism.lol/tracks/rust/exercises/gigasecond/edit

**Steps to reproduce**

1. Copy-Paste the .meta/example.rs into the editor
2. Run the tests

```
We received the following error when we ran your code:
WARNING: student did not upload Cargo.lock. This may cause build errors.
error: no matching package named `chrono` found
location searched: registry `https://github.com/rust-lang/crates.io-index`
required by package `gigasecond v2.0.0 (/mnt/exercism-iteration)`
As a reminder, you're using offline mode (--offline) which can sometimes cause surprising resolution failures,
if this error is too confusing you may wish to retry without the offline flag.
```