bump OCaml (also to rebuild everything after the recent OPAM fix)

buildjob

@@ -28,7 +28,7 @@ set -eo pipefail
 
 status "[buildjob] Using CI branch $(cd ci && git rev-parse --abbrev-ref HEAD) ($(cd ci && git rev-parse HEAD))"
 
-OCAML=${OCAML:-ocaml-base-compiler.4.07.1}
+OCAML=${OCAML:-ocaml-base-compiler.4.08.1}
 TIMING_PROJECT=${TIMING_PROJECT:-$CI_PROJECT_NAME}
 TIMING_CONF=${TIMING_CONF:-$CI_JOB_NAME}