replace-tar-coreutils: Set dpkg's PATH

APT's commit 806e94dcd "Set PATH=/usr/sbin:/usr/bin:/sbin:/bin when
running dpkg", released in 1.8.0_alpha3~1, causes Dpkg to not find the
copies of `rm` and `tar` we set aside in /usr/local/bin.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>