aur-build: pass PKGDEST directly to makepkg
aur-build: pass PKGDEST directly to makepkg

This was done after some concern that PKGDEST may be used by other
programs (in particular, build systems) besides makepkg. However, the
variable is defined in makepkg(8) as being possible part of the
environment, and the custom makepkg.conf parser has broken before (i.e.
a newline was missing in the last concatenated makepkg.conf file).

As this problematic mostly arised due to yaourt exporting every variable
in makepkg.conf (BUILDDIR included), restore the original behavior. See
also:

https://wiki.archlinux.org/index.php?title=Talk:AUR_helpers&oldid=590741#Drop_%22Clean_Build%22_Column

AladW/aurutils,AladW/aurutils