[ci] Make triggered dumux-lecture branch a variable

This branch can be overwritten, for example, in a scheduled pipeline
5 jobs for !2819 with feature/ci-lecture-branch-as-variable
latest detached
Status Name Job ID Coverage
  Trigger
manual full-dune-2.7-clang
manual full-dune-2.7-gcc
manual full-dune-master-gcc
manual minimal-dune-2.7-gcc
 
  Downstream Modules
created trigger lecture