feat(coq): Add flambda support

By default, ocaml is not shipped with flambda support, this MR proposes to add the flambda option.

The only CI for theories took 24"30', while others took about 26" (an increase of 6%).

However, this custom version of ocaml is therefore not available in the Nix cache, so the user must compile his own ocaml version, followed by findlib, zarith and coq. (Around 10 minutes to compile)

Feel free to close if you feel this is not relevant after all.