classes/cargo: fix cargo target for linker override

We don't use the native Rust triples but instead use the Yocto triples
so we need to provide this override properly. This typo was introduced
in 5afc0cc471.

classes/cargo.bbclass

@@ -58,7 +58,7 @@ EOF
 		printf "'%s'\n" "$p"
 	done | sed -e 's/$/,/' >> ${CARGO_HOME}/config
 	echo "]" >> ${CARGO_HOME}/config
-	echo "[target.${RUST_HOST_SYS}]" >> ${CARGO_HOME}/config
+	echo "[target.${HOST_SYS}]" >> ${CARGO_HOME}/config
 	echo "linker = '${RUST_TARGET_CCLD}'" >> ${CARGO_HOME}/config
 }