Do not inherit global variables in a triggered pipeline

When test-pipelines (core or system) are triggered, they inherit all variables from the dune-common pipeline and thus also the DUNECI_CMAKE_FLAGS. This leads to the strange effects, that the system pipelines cannot define their own cmake flags and e.g. python bindings are compiled even if they are supposed to be disabled, see, e.g. the libcxx job in https://gitlab.dune-project.org/infrastructure/dune-nightly-test/-/pipelines/75421 that is configured with "nopython" but compiles python bindings.

This MR fixes this issue by explicitly preventing from inheriting global variables.