With --enable-python=fully-internal, where PYTHON_FOR_BUILD is empty, this
executed the "#!/usr/bin/env python" shebang in pack_images.py, using whatever
python would be found (or not) on the system.

With this fixed, there is no need any more for pack_images.py to be executable.

Change-Id: I40ad3769898f3eb711dbe555d4c3faa9e3c1456d
Reviewed-on: https://gerrit.libreoffice.org/50180Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>