d/bootstrap: Avoid a dummy .deb in the / of bootstrap deb

When debian/bootstrap is run, it creates the a .deb with binaries needed for
building. Since the .deb is created in the directory that is being archived, it
leads to a dummy .deb file inside the created final result (at the root of its
file system).

Create the .deb in .. as this is where it is most expected (as dpkg-buildpackage
places the files by default there). Script output directs the user there too:

  dpkg-deb: building package 'kotlin' in '../kotlin_1.3.31+~1.0.1+~0.11.12-1_all.deb'.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Co-authored-by: Phil Morrell <debian@emorrp1.name>