Fix Opam repository in the `.gitlab-ci.yml`

3 jobs for fix-ci-build in 2 minutes and 17 seconds
Status Job ID Name Coverage
  Test
passed #30974139
ocaml:4030

00:02:11

passed #30974140
ocaml:4042

00:02:11

passed #30974142
ocaml:4050

00:02:15