Merge pull request #36 from dra27/tweak-ci #35

Job	Run time
build (macos-latest, 4.06.x)	19m 25s
build (macos-latest, 4.14.x)	11m 18s
build (macos-latest, 5.1.x)	13m 30s
build (macos-latest, ocaml-variants.5.2.0+trunk)	17m 8s
build (ubuntu-latest, 4.06.x)	12m 33s
build (ubuntu-latest, 4.14.x)	8m 55s
build (ubuntu-latest, 5.1.x)	12m 44s
build (ubuntu-latest, ocaml-variants.5.2.0+trunk)	9m 26s
build (windows-latest, 4.06.x)	4m 49s
build (windows-latest, 4.14.x)	5m 53s
	1h 55m 41s

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