System requirements: The external language toolchains are optional and
each is only required by its matching targets constructor: a
Python installation for tar_target_py(), a Julia installation
for tar_target_jl(), and a Rust toolchain with Cargo for
tar_target_rs() (on Windows, the GNU toolchain, to match R's
ABI). A pipeline that uses a single language needs only that
language's toolchain, and a pipeline that uses none of these
constructors needs no external toolchain at all.