mitigate `dune: command not found
$ dune build archiver/main.exe server/main.exe
/scripts-31185515-6155754304/step_script: line 163: dune: command not found
example : (Job Failed #6155754304) Job #6155754304 failed for 594a1d50:
only happens on GCP, while it's ok on AWS runners
=> mitigate by adding before_script: - eval $(opam env)