store `node_modules.tar.gz` in a temp dir

[rbuckheit]

when we're saving the server response to node_modules.tar.gz, let's save it to a temp directory outside of the source code repository. Doing so will help with certain vagrant configurations where we sync source code between source and guest. As-is now, the saved tarball is synced back to the host while downloading which (might) be causing a perf penalty.