add git-buildpackage/lintian job with .deb artifact
This is based on my work to make a standard gitlab CI image for any
package based on git-buildpackage. The goal is ultimately to make
this the standard gitlab CI image for salsa.debian.org. It currently
automatically installs Build-Depends, runs
gbp buildpackage -uc -us
then runs lintian. Soon it will also auto-detect packages set up for
autopkgtest and run them.