lang/rust-nightly: unbreak cargo git fetch after r507928

libgit2-sys 0.8.2 switched to libgit2 0.29 API without bumping minor
version and adjusting pkg-config check. devel/libgit2 is still at
0.28, so FFI silently fails with a confusing error e.g.,

invalid version 3 on git_proxy_options; class=Invalid (3)

USES=cargo consumers are affected but none use libgit2-sys 0.8.2 yet.

PR: 239785
Reported by: yuri