This is needed when package has dependencies in experimental.
Example for samba:
before_script:
- echo 'deb http://deb.debian.org/debian experimental main' > /etc/apt/sources.list.d/experimental.list
- "echo 'Package: ldb-tools libldb* python*-ldb*\nPin: release a=experimental\nPin-Priority: 500' > /etc/apt/preferences.d/experimental.pref"
Fixes: pipeline#24 (closed).