deb-pkg: Fix for relative paths

When $srctree or $objtree are relative paths, we cannot change directory
and refer to them in the same subshell. Do the redirection outside of
the subshell to fix this.

Reported-and-tested-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>