Ensure venv pex does not enter a re-exec loop. (#1286)
Ensure venv pex does not enter a re-exec loop. (#1286)

Use the same environment variable trick used by pex_bootstrapper to
ensure we never enter aninifinite re-exec loop. Also check more valid
venv python path variants by resolving symlinks in the parent
directories of the known valid pythons.

Fixes #1285
pantsbuild/pex,jsirois/pex,pantsbuild/pex,jsirois/pex