Fix issue 668: make release.sh execute packaged pants without loading in...

...ternal 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.