Add nodejs to PATH explicitly when compiling npm

Hello!

Somewhere in the npm compile process, the `node` executable is called. This is problematic if node['nodejs']['dir'] is set somewhere outside the normal $PATH.

This commit explicitly adds the nodejs compilation path when npm is installed.

I tried running the test-kitchen suite (and writing a test for this), but I couldn't get `master` to build. :(. Let me know if you can think of a good way to test this kind of thing.

Cheers!