Better timer (fix build on mac) #4

Job	Run time
install (dev, Ubuntu, ubuntu-latest, 4.09.1)	6h 0m 14s
install (dev, macOS, macos-latest, 4.14.2)	6h 0m 16s
install (dev, Windows, windows-latest, 4.13.1, git+https://github.com/ocaml/ocamlfind.git#master,...	6h 0m 9s
install (8.20.0, Ubuntu, ubuntu-latest, 4.09.1)	17m 12s
install (8.20.0, macOS, macos-latest, 4.14.2)	15m 23s
install (8.20.0, Windows, windows-latest, 4.13.1, git+https://github.com/ocaml/ocamlfind.git#mast...	29m 44s
install (8.19.0, Ubuntu, ubuntu-latest, 4.09.1)	17m 54s
install (8.19.0, macOS, macos-latest, 4.14.2)	15m 18s
install (8.19.0, Windows, windows-latest, 4.13.1, git+https://github.com/ocaml/ocamlfind.git#mast...	30m 11s
install (8.18.0, Ubuntu, ubuntu-latest, 4.09.1)	17m 48s
install (8.18.0, macOS, macos-latest, 4.14.2)	13m 39s
install (8.18.0, Windows, windows-latest, 4.13.1, git+https://github.com/ocaml/ocamlfind.git#mast...	29m 53s
	21h 7m 41s

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