trivago#35

based on the changes suggested in that PR but apparently not further evaluated, this change should do what is required, even without all the steps in between mentioned here https://github.com/trivago/parallel-webpack/pull/35/files/bbdc8ffa14a2aa81adcdf5e1b5a5f1b951ffd215#r108773189

process.argv is in several locations overwritten by the args passed to the initial run.js and therefore always contain the required args so we just need to filter them, adjust them to kv pairs and then reduce them to an object to be passed as the env object