cargo.bbclass: improve debug note showing paths

There was a bbnote that showed the path to the rustc and cargo binaries
being used but this was not necessarily correct since it didn't use the
exact variables used.

(cherry picked from commit c55fb403f8)

classes/cargo.bbclass

@@ -64,8 +64,8 @@ CARGO_TARGET_SUBDIR="${HOST_SYS}/release"
 oe_cargo_build () {
 	export RUSTFLAGS="${RUSTFLAGS}"
 	export RUST_TARGET_PATH="${RUST_TARGET_PATH}"
-	bbnote "cargo = $(which cargo)"
-	bbnote "rustc = $(which rustc)"
+	bbnote "cargo = $(which ${CARGO})"
+	bbnote "rustc = $(which ${RUSTC})"
 	bbnote "${CARGO} build ${CARGO_BUILD_FLAGS} $@"
 	"${CARGO}" build ${CARGO_BUILD_FLAGS} "$@"
 }