debian: Switch from pbuilder to cowbuilder

For each package it builds, pbuilder creates a new chroot by unpacking
a tarball.    cowbuilder is a wrapper around pbuilder that creates
copy-on-write chroot directories, avoiding the need to unpack the tarball.

Although unpacking the tarball isn't extremely slow, this change should
give us back a few seconds per package.

Signed-off-by: Euan Harris <euan.harris@citrix.com>