Use separate build directories for Rust libraries in dune

Currently these all use default/target, which seems to cause
difficult-to-debug race conditions in OSS CI.
Strangely, (locks /cargo) is supposed to prevent this given that locks
with an absolute path is documented by dune to be a global lock, but it
doesn't seem to work as intended.

Author: mszabo-wikia

hphp/hack/scripts/invoke_cargo.sh

@@ -40,7 +40,7 @@ if [ -z "${HACK_NO_CARGO_VENDOR}" ]; then
 fi
 
 if [ -z "${TARGET_DIR}" ]; then
-  TARGET_DIR="${HACK_BUILD_ROOT}/target"
+  TARGET_DIR="${HACK_BUILD_ROOT}/target/$pkg"
 fi
 
 if [ -z ${HACKDEBUG+1} ]; then