Running `cargo creusot` twice on the same project fails

[xldenis]

There's a problem introduced by @voidc's MIR_BODIES patch where running cargo creusot twice on the same program causes it to crash saying: thread 'rustc' panicked at 'called Option::unwrap()on aNone value', creusot/src/ctx.rs:194:62.

[xldenis]

This can be worked around by systematically running cargo clean.

Perhaps a detail which is relevant is that this crashes on the build.rs file, which is annoying that its even compiled by creusot.