How do I invoke Host/Install

I am trying to use the Rustup package developed by @Grommish, in a 18.06.5 buildroot on Ubuntu 20.04, but I don't think my question is package-specific. This package has a Host/Install target that I want to invoke by itself, but I don't know how. I know I can do eg.

make package/rustup/host/compile invoke the Host/Compile target. But when I try:

make package/rustup/host/install

...I get:

make[1]: *** No rule to make target 'package/rustup/host/install'.  Stop.

How do I invoke the host tool install target?