Nixpkgs: npm link fails due to read-only fs

[Havvy]

Issue description

The location that npm link tries to install symlinks into is inside of it's /nix installation directory which is unwritable, and thus fails. Specifically, when trying to link a package named foo, it tries to create a symlink at /nix/store/<nodejs-nix-package-hash>-nodejs-<nodejs-version>/lib/node_modules/foo.

It should try writing (and reading) to somewhere else that's actually allowed to be written to.

Steps to reproduce

Do npm link inside a node.js package.

If you don't have a local node.js package:

1. Install Node.js. (nix-env -iA nixpkgs.nodejs)
2. Create a directory, and cd into it.
3. npm init to create a package.json.
4. npm link

[dejanr]

It is possible to make a workaround for this by having a custom prefix inside ~/.npmrc. So if you create ~/.npmrc with content:

  prefix=~/.npm

Next time your run npm link or npm install -g, it would use ~/.npm as root folder.

It would be nice to incorporate this inside nodejs nix expression so it could use some global writable folder by default. I would take a closer look if we could have this in nodejs expression.