Fix CI

[vouillon]

No description provided.

[vouillon]

I don't understand why this still fails with Nix.

[rgrinberg]

I think it might be because we're building the benchmarks. The error message says:

File "benchmarks/dune", line 8, characters 7-22:
8 |   (not %{env:CI=false}))
           ^^^^^^^^^^^^^^^
Error: Only architecture, system, model, os_type, ccomp_type, profile,
ocaml_version, context_name and arch_sixtyfour variables are allowed in this
'enabled_if' field. Please upgrade your dune language to at least 3.15.

So I'd say give that a try and bump the dune version in the project file to 3.15

[vouillon]

So I'd say give that a try and bump the dune version in the project file to 3.15

Well, what's puzzling is that I have already bumped the dune version there.