Merge branch 'feature/ci/ocaml-4.12.1-dune-3-env' into 'feature/bobot/jbuilder'

[ci] Fix OCaml version to 4.12 for Dune

See merge request frama-c/meta!60

ci.sh

@@ -76,4 +76,4 @@ cleanup () {
 trap cleanup EXIT
 
 # run the build
-nix-build --no-out-link "$curdir/nix/pkgs.nix" $OPTS -A ocamlPackages.meta
+nix-build --no-out-link "$curdir/nix/pkgs.nix" $OPTS -A ocaml-ng.ocamlPackages_4_12.meta