Fix issue 668: make release.sh execute packaged pants without loading internal backends during testing

The introduction of src/python/internal_backend plugins broke build-support/bin/release.sh.
Release.sh, during dry-run and real testing, fails to execute packaged pants, because it
doesn't have internal_backend packaged within but the repo root pants.ini still asks it
to load them.

The fix is to use PANTS_CONFIG_OVERRIDE during such execution with an empty plugin list.

Testing Done:
ran and passed:

- build-support/bin/release.sh -n
- build-support/bin/release.sh -t

CI passed: https://travis-ci.org/jinfeng/jinfeng-pants-fork/builds/37896390

Bugs closed: 668

Reviewed at https://rbcommons.com/s/twitter/r/1158/