support having `rust_target` track rust-toolchain.toml

I recently ran into an issue on a repo that has rustc pinned to 1.81 with a rust-toolchain.toml file because of a regression in 1.82.

bindgen on that repo was generating bindings for 1.83, which included output that 1.81 doesn’t understand.

Adding rust_target(RustTarget::stable(81, 0)) was easy enough once I discovered it, but once 1.85 (which fixes the regression) is stable, we’ll update rust-toolchain.toml to point at that release, at which point we’ll have to update build.rs with rust_target(RustTarget::stable(85, 0)).

It would be great if there were something like RustTarget::toolchain_toml() to indicate that it should extract the target version from that file. It feels like that should maybe even be the default (if such a file exists), but I’m happy either way.