lang/rust-nightly: enable the Rust profiler

Enable the rust profiler on rust-nightly (as an unstable feature, it's not
allowed on the stable toolchain). This allows it to generate code coverage
reports.

https://doc.rust-lang.org/unstable-book/compiler-flags/profile.html
https://crates.io/crates/grcov

Reviewed by: tobik
Approved by: tobik (maintainer)
Differential Revision: https://reviews.freebsd.org/D24765