rust/hooks: ensure the build output ends up in the right place

Otherwise cargoInstallHook can fail to find and actually install it.

pkgs/build-support/rust/hooks/cargo-build-hook.sh

@@ -6,6 +6,9 @@ cargoBuildHook() {
     runHook preBuild
 
     if [ ! -z "${buildAndTestSubdir-}" ]; then
+        # ensure the output doesn't end up in the subdirectory
+        export CARGO_TARGET_DIR="$(pwd)/target"
+
         pushd "${buildAndTestSubdir}"
     fi