Install commands uses wrong variable TARGET_SYS instead of RUST_TARGET_SYS

[Dvergatal]

Generally this works in kirkstone but https://git.openembedded.org/openembedded-core/tree/meta/classes-recipe/rust-common.bbclass#n110 defines alredy a proper variable so does the https://github.com/meta-rust/meta-rust/blob/master/classes/rust-common.bbclass#L123 and if we are switching to newest yocto release it stops to build...