ci: try to fix pbs i don't understand

.gitlab-ci.yml

@@ -22,6 +22,7 @@ test_opam_test:
   stage: test
   script:
     - sudo apt-get install -y m4
+    - opam repo add opam-ocaml-org "https://opam.ocaml.org"
     - opam repo add verimag-sync-repo "http://www-verimag.imag.fr/DIST-TOOLS/SYNCHRONE/test/opam-repository"
     - opam update
     - opam depext  -y lutin