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

5 jobs for !2867 with fix/make-install-script
latest detached
Status Name Job ID Coverage
  Trigger
passed full-dune-latest-release-clang

passed full-dune-latest-release-gcc

passed full-dune-master-gcc

passed minimal-dune-latest-release-gcc

 
  Downstream Modules
passed trigger lecture