lang/rust-nightly: Update to 1.43.0.20200306

libgit2-sys is now based on libgit2 0.99.0. While 0.99.0 is supposed
to be API compatible with 0.28, it has not yet been updated in the
ports tree and the git2 crate now make use of some of the new
function return values. Stop relying on system libgit2 and use the
bundled one. Upstream does not make any guarantees about system
libgit2 support in the first place.

Changes: https://github.com/rust-lang/rust/compare/6d69caba11...96bb8b31c8