rust: put host build dir to target build dir

rust: put host build dir to target build dir

When user runs `make clean` command, everything in `$(STAGING_DIR)`
(where we installed rust) will be removed, but `$(BUILD_DIR_HOST)`
(where we compiled rust and stored build stage) is untouched.

So when user starts a new build after that, OpenWrt buildroot will
still consider `rust` is installed already, resulting the build error
"cargo: command not found".

Fix this by moving to target build dir as well.

Fixes: f489e019ac4a ("rust: compile host package per target")
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>