pakcage/pkg-cargo: use post-process opts to pass manifest path

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

package/pkg-cargo.mk

@@ -192,7 +192,7 @@ $(2)_DL_ENV += CARGO_HOME=$$(BR_CARGO_HOME)
 
 # If building in a sub directory, use that to find the Cargo.toml
 ifneq ($$($(2)_SUBDIR),)
-$(2)_DL_ENV += BR_CARGO_MANIFEST_PATH=$$($(2)_SUBDIR)/Cargo.toml
+$(2)_DOWNLOAD_POST_PROCESS_OPTS += -m$$($(2)_SUBDIR)/Cargo.toml
 endif
 
 # Because we append vendored info, we can't rely on the values being empty