Change dpkg-build flags to sign source deb
Change dpkg-build flags to sign source deb

Don't include -us -uc when creating deb source and dpkg will
actually sign the source package.

Signed-off-by: Ryan Harper <7c8ea25f69f5166c51151e84bf5db678c2384828@canonical.com>

CanonicalLtd/subiquity,CanonicalLtd/subiquity