Ring has duplicate adjacent points when fastforwarding in tutorial mode

**Steps to Reproduce**

1. Run `cargo run --bin game`
2. Start traffic simulation tutorial
3. Fast forward to tutorial step 7/9
4. Click "Try It"
5. Click the "jump to specific time" button
6. Observe crash

**Log Output**
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Ring has duplicate adjacent points near Pt2D(500, 50)', geom\src\ring.rs:73:24

[abstreet_ring_crash.txt](https://github.com/a-b-street/abstreet/files/11362440/abstreet_ring_crash.txt)

**Expected Behaviour**
Tutorial fast forwards successfully with no crash