[bin][instscript] use bracket operator over get()

5 jobs for !2867 with fix/make-install-script
detached
Status Name Job ID Coverage
  Trigger
manual full-dune-latest-release-clang
manual full-dune-latest-release-gcc
manual full-dune-master-gcc
manual minimal-dune-latest-release-gcc
 
  Downstream Modules
created trigger lecture