test(melange): use a different flag for forwards-compat #29312

Job	Run time
Build microbenchmarks	2m 38s
Create local opam switch (macos-latest, 5)	5m 19s
Coq 8.16.1	7m 9s
Create local opam switch (macos-latest, 4.14)	6m 13s
Build (macos-latest, 5.1.x)	14m 10s
Documentation	57s
Create local opam switch (ubuntu-latest, 5)	5m 3s
Build (ubuntu-latest, 5.1.x)	9m 52s
Format	1m 5s
Create local opam switch (ubuntu-latest, 4.14)	4m 59s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	1m 39s
Build monorepo benchmark docker image	3m 40s
Build (4.14.x, ubuntu-latest, true)	1m 28s
Nix (macos-latest)	2m 0s
Build (4.14.x, macos-latest, true)	2m 38s
Wasm_of_ocaml	10m 33s
Nix (ubuntu-latest)	52s
Build (5.1.x, macos-13, true)	3m 53s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 34s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 17s
Build (4.08.x, ubuntu-latest, true)	1m 32s
Build (4.04.x, ubuntu-latest, true, true)	4m 44s
Build (4.02.x, ubuntu-latest, true)	3m 54s
	1h 57m 9s

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