Figure out why cargo rebuilds everything unnecessarily.

cargo run --bin rav1e --release
   Compiling rav1e v0.1.0 (file:///Users/.../Workspace/rav1e)
    Finished release [optimized] target(s) in 6.98 secs
     Running `target/release/rav1e`

7s is too long to wait if the code hasn't changed.