Recent development versions of makepkg support reproducible builds
through the environment variable SOURCE_DATE_EPOCH. Pass this variable
through makechrootpkg to makepkg when available.

Also initialize SOURCE_DATE_EPOCH whenever running archbuild to enforce
reproducible builds for repository packages.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Levente Polyak <anthraxx@archlinux.org>