Please add variable GBP_BUILDPACKAGE_EXTRA_ARGS to add custom switches to git-buildpackage

I'm using the overlay layout (only debian/ in Git). Building using Gitlab CI now works fine. But I have to use debian/gbp.conf to set tarball_dir to '..', otherwise the build fails although the tarball is there (either received via origtargz or uscan). However, tarball_dir or preexport are usually variables I don't want to define in debian/gbp.conf, because they might differ on every maintainers system. Therefor it would be nice to have a variable like GBP_BUILDPACKAGE_EXTRA_ARGS (similar to REPROTEST_EXTRA_ARGS) to define these for the build using CI. My configs:

https://salsa.debian.org/debichem-team/shelxle/blob/gitlab_ci/debian/gbp.conf https://salsa.debian.org/debichem-team/shelxle/blob/gitlab_ci/debian/gitlab-ci.yml

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