Stagger dependabot update times (#1868) #855

Job	Run time
install (dev, 4.11.1, Ubuntu, ubuntu-latest)	40m 46s
install (dev, 4.11.1, macOS, macos-latest)	50m 31s
install (dev, 4.11.1, Windows, windows-latest, -async-proofs-j 1, opam-repository-mingw: https://...	1h 37m 59s
install (8.19.0, 4.11.1, Ubuntu, ubuntu-latest)	40m 22s
install (8.19.0, 4.11.1, macOS, macos-latest)	56m 14s
install (8.19.0, 4.11.1, Windows, windows-latest, -async-proofs-j 1, opam-repository-mingw: https...	1h 38m 54s
install (8.18.0, 4.11.1, Ubuntu, ubuntu-latest)	41m 52s
install (8.18.0, 4.11.1, macOS, macos-latest)	1h 17m 57s
install (8.18.0, 4.11.1, Windows, windows-latest, -async-proofs-j 1, opam-repository-mingw: https...	1h 43m 36s
	10h 8m 11s

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