This fixes a bug whereby uses of 'package-with-python2' at the top-level
in modules other than (gnu packages python) could lead to an "Unbound
variable: python2" error due to the circular references.
Reported by Tomáš Čech.
* guix/build-system/python.scm (package-with-explicit-python)[arguments]:
Check whether PYTHON is a promise, and force it if it is.
(package-with-python2): Wrap 'default-python2' call in 'delay'.
8c9653b373