refactor: expand Stdlib.Filename in Stdune #38083

Job	Run time
Nix Tests (ubuntu-latest)	8m 31s
Documentation	57s
Format	1m 32s
Cygwin Build	3m 24s
Build with OxCaml (ubuntu-latest)	26m 2s
Nix Build 4.08 (ubuntu-latest)	5m 56s
Test that the utop dev tool can be built and run	9m 6s
Nix Build (macos-latest)	2m 36s
Nix Build (ubuntu-latest)	1m 12s
Bootstrap (ubuntu-latest)	4m 38s
Rocq 9.1 (native)	13m 51s
Create local opam switch (ubuntu-latest, 5)	4m 51s
Create local opam switch (macos-latest, 5)	8m 23s
Building Dune with OxCaml	33m 25s
Wasm_of_ocaml	7m 38s
Build microbenchmarks	1m 55s
Rocq 9.1	7m 48s
Build (ocaml-base-compiler.5.4.0,system-mingw, windows-latest, ocaml-compiler.5.4.0-system-mingw,...	10m 54s
Build (ocaml-compiler.5.4.0,system-msvc, windows-latest, ocaml-compiler.5.4.0-system-msvc, true)	10m 1s
Build (ocaml-variants.5.5.0+trunk, ocaml-variants.5.5.0+trunk, ubuntu-latest)	1m 36s
Build (5.4.x, 5.4.x, macos-15-intel)	4m 56s
Build (4.14.x, 4.14.x, macos-latest)	3m 24s
Build (5.4.x, 5.4.x, ubuntu-latest, true)	6m 35s
Build (ocaml-variants.4.14.2+options,ocaml-option-32bit, ocaml-variants.4.14.2-ocaml-option-32bit...	4m 9s
Build (4.14.x, 4.14.x, ubuntu-latest)	1m 11s
Build (5.4.x, 5.4.x, macos-latest, true)	15m 37s
	3h 20m 8s

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