archive-zip: use a local variable to store the creator version

archive-zip: use a local variable to store the creator version

Use a simpler conditional right next to the code which makes a higher
creator version necessary -- namely symlink handling and support for
executable files -- instead of a long line with a ternary operator.
The resulting code has more lines but is simpler and allows reuse of
the value easily.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>