Stop stomping on $HOME; use npm_config_prefix

`npm_config_prefix` is ["The location to install global items."][npm-config]
It is used by `global` and other settings to define the place in which the
global packages and config settings should go.

`$HOME` is used by `curl` to locate important configuration files, and
since `npm` uses `git` which uses `curl`, it's helpful to make sure that
`$HOME` remains intact during the execution of `npm`.

[npm-config]: https://npmjs.org/doc/config.html