CI: Make feature branch pipelines manual

A branch pushed before its merge request exists gets a branch pipeline that the merge request pipeline created moments later cannot supersede, because the two are different refs (see !1048 (merged)). Such a branch pipeline now has every job manual, so it costs nothing and can still be started by hand when CI is wanted before the MR: linux:build-sdist first, as the test and documentation jobs need its artifact. Merge request pipelines, the long lived branches, tags and schedules are unchanged.