If pkg_nofetch needs to be spawned inside fetch() and it happens that

If pkg_nofetch needs to be spawned inside fetch() and it happens that
PORTAGE_BUILDDIR doesn't exist, like when called by digestgen(), use
mkdtemp to create a private temporary directory so that pkg_nofetch
can be spawned (directory needed to satisfy safe $PWD requirement of
bug #239560). This is more user friendly since before the pkg_nofetch
phase would simply be skipped in this case. Thanks to Petteri Räty
<betelgeuse@g.o> for reporting.

svn path=/main/trunk/; revision=12174