rust: Move CARGO_HOME to $(DL_DIR)/cargo

As CARGO_HOME mainly functions as a download and source cache[1], moving
it into $(DL_DIR) allows it to persist and be reused between different
buildroots/sdks (when DL_DIR is set to a custom/external location).

[1]: https://doc.rust-lang.org/cargo/guide/cargo-home.html

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

lang/rust/rust-values.mk

@@ -5,7 +5,7 @@
 # Rust Environmental Vars
 RUSTC_HOST_SUFFIX:=$(word 4, $(subst -, ,$(GNU_HOST_NAME)))
 RUSTC_HOST_ARCH:=$(HOST_ARCH)-unknown-linux-$(RUSTC_HOST_SUFFIX)
-CARGO_HOME:=$(STAGING_DIR)/host/cargo
+CARGO_HOME:=$(DL_DIR)/cargo
 CARGO_VARS?=
 
 ifeq ($(CONFIG_USE_MUSL),y)