Fix crunchgen usage of mkstemp()

On Glibc systems mkstemp can only be used once with the same template
string since it will be modified in-place and no longer contain any 'X' chars.
It is fine to reuse the same file here but we need to be explicit and use
open() instead of mkstemp() on the second use.

While touching this file also avoid a hardcoded /bin/pwd since that may not
work when building on non-FreeBSD systems.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D25990