Installer does not work when only python3 is available

[dagewa]

Comes from dials/dials#2222; please see there for details

[bkpoon]

Is this the source installer or a binary installer? We had one Phenix user have a similar issue, but it was only for the source installer.

[ndevenish]

cctbx_project/libtbx/auto_build/create_installer.py

Lines 34 to 49 in fbe93bb

	INSTALL_SH = """\
	#!/bin/bash
	if [ -z "$PYTHON_EXE" ]; then
	PYTHON_EXE='/usr/bin/python'
	if [ -f "/usr/bin/python2.7" ]; then
	PYTHON_EXE='/usr/bin/python2.7'
	elif [ -f "/usr/bin/python2.6" ]; then
	PYTHON_EXE='/usr/bin/python2.6'
	elif [ -f "/usr/bin/python2" ]; then
	PYTHON_EXE='/usr/bin/python2'
	elif [ -f "./conda_base/bin/python" ]; then
	PYTHON_EXE='./conda_base/bin/python'
	fi
	fi
	$PYTHON_EXE ./bin/install.py $@
	"""