Avoid setting DEBUG env in .studiorc or builds

I just found that strange additional build output, slow builds, and occasional compilation failures with errors about -fPIC were all caused by my .studiorc filing containing export DEBUG=1 that a node application I was testing under the studio used.

Lesson learned: Do not leak DEBUG=1 into your build/studio environment

If you want to run something with DEBUG set under a studio, consider instead defining a command like this in your .studiorc:

run-app() {
  pushd "/src" > /dev/null
  DEBUG=1 hab pkg exec core/node node myapp.js
  popd > /dev/null