CI: add the official opam repo to get dune 2.0

.gitlab-ci.yml

@@ -12,6 +12,7 @@ build:
   stage: build
   script:
     - sudo apt-get install -y m4
+    - opam repo add official "https://opam.ocaml.org/"
     - opam repo add verimag-sync-repo "http://www-verimag.imag.fr/DIST-TOOLS/SYNCHRONE/test/opam-repository"
     - opam update
     - opam install -y ocamlfind dune extlib