npmHooks.npmConfigHook: add npmRoot option

pkgs/build-support/node/build-npm-package/hooks/npm-config-hook.sh

···
     export NIX_NODEJS_BUILDNPMPACKAGE=1
     export prefetchNpmDeps="@prefetchNpmDeps@"
 
+    if [ -n "${npmRoot-}" ]; then
+      pushd "$npmRoot"
+    fi
+
     echo "Configuring npm"
 
     export HOME="$TMPDIR"
···
 
     rm "$CACHE_MAP_PATH"
     unset CACHE_MAP_PATH
+
+    if [ -n "${npmRoot-}" ]; then
+      popd
+    fi
 
     echo "Finished npmConfigHook"
 }