[!317] set full path without which

Merge branch 'feature/fix-profiling-fullpath' into 'master'

ref:extensions/dune-codegen using which only works, if generate_operators is
in PATH, otherwise cmake fails. This is especially the case if dune-codegen is
configured completely new.

See merge request [extensions/dune-codegen!317]

  [extensions/dune-codegen!317]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/317