build-system/cargo: Set libclang environment variable when available.

build-system/cargo: Set libclang environment variable when available.

* guix/build/cargo-build-system.scm (configure): When clang is available
in the build environment set the LIBCLANG_PATH variable.
* gnu/packages/crates-graphics.scm (rust-aom-sys-0.1,
uust-dav1d-sys-0.3),
* gnu/packages/crates-io.scm (rust-bindgen-0.55, rust-bindgen-0.54,
rust-bindgen-0.53, rust-bindgen-0.52, rust-cexpr-0.4, rust-cexpr-0.3,
rust-cexpr-0.2, rust-clang-sys-1, rust-clang-sys-0.29,
rust-clang-sys-0.26, rust-clang-sys-0.22, rust-clang-sys-0.11,
rust-libpijul-0.12, rust-nettle-7, rust-nettle-sys-2,
rust-sequoia-openpgp-0.9)[arguments]: Remove phases which set an
environment variable to find clang.
[inputs]: Rename instances of libclang with clang.