gitlab-ci: Build package

We unfortunately can't use salsa-ci's prebuilt pipeline since that hangs on large jobs: salsa/support#180 (closed)

We redirct output to a file to work around: salsa/support#191 (closed)

This builds the package only, i'll add a lintian and autopkgtest step separately since this needs some more tweaking with this instances current limits and compile test already helps a lot. I'm doing this against debian/sid on purpose since the build there fails for 6.0.0. I'll cherry pick it over once things build again.