chore(ci): test FFI on CI using nightly
chore(ci): test FFI on CI using nightly

Now that FFI is opt-in using cargo's --crate-type, nightly needs to be used
on CI: this flag is still unstable and requires a nightly to enable -Z unstable options.

tshepang/hyper,hyperium/hyper,hyperium/hyper,tshepang/hyper,hyperium/hyper,tshepang/hyper