gbuild: More crude hacking to fix over-evaluation of gb_Python_PRECOMMAND

...in the spirit of 3e70e26cbc96667e2968cd325737053bf8bffb78 "gbuild: fix over-
evaluation in gb_ExternalExecutable__set_internal"

Change-Id: I3d5859bd3437cc050250e66d723d3dee3056ef46
--enable-python=fully-internal to force PYTHON_FOR_BUILD=

(for testing purposes, mainly)

Change-Id: I50d3c92b9ac0cc9dda55b7340f657acb74f675f6