$HOST_SYS Rust target is not recognized by Rust provided by oe-core and
meta-lts-mixins layers, the correct one should be $RUST_HOST_SYS.
Fixes the following compilation error:
error: Error loading target specification: Could not find specification
for target "aarch64-poky-linux". Run `rustc --print target-list`
for a list of built-in targets
Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>