Fix building ocamlPackages_flambda2 (#1835) #5469

Job	Run time
nix-fmt-check	1m 39s
Native packages (Linux, OCaml 4_14)	2m 15s
Native packages (Linux, OCaml 5_1)	2m 16s
Native packages (Linux, OCaml 5_2)	2m 31s
Native packages (Linux, OCaml 5_3)	3m 42s
Native (Top-level) packages (ubuntu-latest)	1m 22s
Native packages (macOS (macos-13), OCaml 5_1)	24m 47s
Native (Top-level) packages (macos-13)	5m 10s
Native (Top-level) packages (macos-14)	2m 11s
arm64 packages (OCaml 4_14)	1m 35s
musl packages (OCaml 4_14)	1m 36s
arm64 packages (OCaml 5_1)	1m 24s
Native packages (macOS (macos-13), OCaml 5_2)	20m 28s
musl packages (OCaml 5_1)	1m 33s
Native packages (macOS (macos-13), OCaml 5_3)	24m 55s
arm64 packages (OCaml 5_2)	1m 36s
Native packages (macOS (macos-14), OCaml 4_14)	4m 10s
musl packages (OCaml 5_2)	1m 33s
Native packages (macOS (macos-14), OCaml 5_1)	1s
arm64 packages (OCaml 5_3)	1m 44s
Native packages (macOS (macos-14), OCaml 5_2)	8m 5s
musl packages (OCaml 5_3)	1m 37s
Native packages (macOS (macos-14), OCaml 5_3)	18m 6s
	2h 14m 16s

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