refactor: use stdune in diagnostics #29495

Job	Run time
Build (macos-latest, 5.1.x)	1s
Format	1m 4s
Nix (macos-latest)	1m 53s
Nix (ubuntu-latest)	54s
Build microbenchmarks	2m 27s
Build (ubuntu-latest, 5.1.x)	8m 43s
Create local opam switch (macos-latest, 5)	2m 32s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	1m 0s
Build monorepo benchmark docker image	3m 44s
Create local opam switch (macos-latest, 4.14)	2m 32s
Wasm_of_ocaml	10m 21s
Build (4.14.x, ubuntu-latest, true)	1m 39s
Create local opam switch (ubuntu-latest, 5)	5m 4s
Build (4.14.x, macos-latest, true)	29s
Create local opam switch (ubuntu-latest, 4.14)	4m 57s
Build (5.1.x, macos-13, true)	4m 25s
Documentation	58s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 25s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 12s
Build (4.08.x, ubuntu-latest, true)	2m 19s
Coq 8.16.1	7m 7s
Build (4.04.x, ubuntu-latest, true, true)	4m 31s
Build (4.02.x, ubuntu-latest, true)	4m 0s
	1h 33m 17s

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