ci: one more try to fix it

.gitlab-ci.yml

@@ -11,6 +11,7 @@ stages:
 build:
   stage: build
   script:
+    - opam repo add http https://opam.ocaml.org/
     - opam repo add verimag-sync-repo "http://www-verimag.imag.fr/DIST-TOOLS/SYNCHRONE/test/opam-repository"
     - opam update
     - sudo apt update