dune: use build_if instead of workaround #236

Job	Run time
Tests (ubuntu-latest, 4.03.0)	10m 26s
Tests (ubuntu-latest, 4.04.0)	10m 9s
Tests (ubuntu-latest, 4.05.0)	10m 22s
Tests (ubuntu-latest, 4.06.0)	10m 40s
Tests (ubuntu-latest, 4.07.0)	9m 1s
Tests (ubuntu-latest, 4.08.0)	7m 30s
Tests (ubuntu-latest, 4.09.0)	7m 45s
Tests (ubuntu-latest, ocaml-variants.4.11.2+fp+flambda)	8m 17s
Tests (ubuntu-latest, 4.11.1)	7m 24s
Tests (ubuntu-latest, 4.12.0)	6m 54s
Tests (ubuntu-latest, 4.13.1)	6m 48s
Tests (ubuntu-latest, 4.14.0)	7m 14s
Tests (ubuntu-latest, 5.0.0)	7m 6s
Tests (ubuntu-latest, 5.1.0)	7m 3s
Tests (ubuntu-latest, 5.2.0)	7m 19s
Tests (windows-latest, 4.13.1)	3m 43s
Tests (macos-latest, 4.13.1)	6m 50s
	2h 14m 31s

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