enforce manual run of test-build-twice job by default

The test-build-twice job adds another two package builds and their associated resource costs to every pipeline run for little gain. This merge requests marks the job with when: manual so that it can be triggered by the developer when they want to make sure that their clean target works correctly at their discretion (when they make changes that are likely to break the clean target, right before a new upload or just once per release cycle).

The clean target will not break often for common packages. For those packages where the maintainer feels that the clean target will break often, they can change when: manual to when: always in their debian/salsaci.yml.

Some more comments on this topic here: #193 (comment 422553)

Essentially, this merge request turns the build-twice job from an opt-out into an opt-in.