Retire staged Yarn dependencies during Nix installation (#473)

yarn-dependencies.nix

@@ -9,10 +9,18 @@ let
     overrideAttrs = _: {
       buildPhase = "runHook preBuild; runHook postBuild";
       installPhase = ''
+        set -o pipefail
         runHook preInstall
         test -d node_modules
+        test ! -L node_modules
         mkdir -p "$out"
-        cp -a node_modules/. "$out/"
+        # Installation is finished: retire its disposable archive cache, then
+        # stream dependencies across the build/store mount boundary without
+        # retaining a second complete tree. This is the same installation
+        # policy used by Central's dependency derivations.
+        rm -rf .yarn/cache
+        ${pkgs.gnutar}/bin/tar --create --file=- --remove-files node_modules \
+          | ${pkgs.gnutar}/bin/tar --extract --file=- --strip-components=1 --directory="$out"
         # Node resolves installed packages directly. This generated index is for
         # Yarn's linker, not a runtime input; do not export build-session state.
         rm -f "$out/.yarn-state.yml"