checkPlugin: Use absolute path when installing `ep_etherpad-lite`

This makes it possible to check plugins that were installed by
symlinking into `node_modules/` like this:

    git clone git@github.com:ether/etherpad-lite.git
    git clone git@github.com:ether/ep_example.git
    cd etherpad-lite
    npm i ep_example@file:../ep_example
    node ./bin/checkPlugin.js ep_example