archbuild: exec makechrootpkg to preserve exit value

This fixes a compound command such as the one below from continuing even
if the first fails.

  extra-x86_64-build && extra-i686-build

The problem is that 'cleanup 0' is triggered in archbuild even after an
unsucessful call to makechrootpkg. Since both archbuild and
makechrootpkg share the exact same cleanup function (from lib/common),
we simply force the shell to exit with the true return value by exec'ing
the call to makechrootpkg.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>