Non-Rust artifact dependencies for build scripts

As detailed in this larger piece surveying build.rs scripts, a lot of build.rs scripts that generate FFI bindings (and thus need to pull in external sources) or need to pull in external corpuses of information (e.g. NLP-oriented crates) often need to download external artifacts themselves. By artifacts, I mean: remote HTTP archives or Git repositories on-disk paths to executables the build.rs needs to shell out to and expects to be in $PATH on-disk paths to foreign library information the bui...