Use and process.env.INIT_CWD to find path to app for npm5 local installs

[PanayotCankov]

Installing from local directory, npm will set process.env.INIT_CWD to the app path, and symlink to the postinstall scripts will be resolved in __dirname

[sis0k0]

Currently, if you are using npm 5 and try to install a nativescript plugin with hooks from local path, the hooks it contains, won't be copied to the project directory. That's because npm install creates a symlink for the installed plugin (again, npm5+). As a result, if you execute a livecycle script, located inside the plugin dir (for ex. - ~/my-plugin/postinstall.js), the value of __dirname will be the original location of the plugin on the drive (~/my-plugin), instead of ~/my-app/node_modules/my-plugin.
INIT_CWD is available in npm since v5.3 and it contains the path where you spawned the process from.
In short, if you run npm install inside ~/my-app, INIT_CWD will be assigned to ~/my-app.
This change fixes the mentioned problem with the hooks.