Adapt to coq/coq#19690 (Hint Extern follows proof mode) (#2058) #993

Job	Run time
install (dev, macOS, macos-latest, 4.14.2)	38m 52s
install (dev, Windows, windows-latest, 4.13.1, -async-proofs-j 1, git+https://github.com/ocaml/oc...	1h 33m 1s
install (8.20.0, Windows, windows-latest, 4.13.1, -async-proofs-j 1, git+https://github.com/ocaml...	1h 33m 41s
install (dev, Ubuntu, ubuntu-latest, 4.09.1)	41m 21s
install (8.20.0, macOS, macos-latest, 4.14.2)	35m 33s
install (8.19.0, Ubuntu, ubuntu-latest, 4.09.1)	41m 14s
install (8.19.0, macOS, macos-latest, 4.14.2)	36m 53s
install (8.18.0, macOS, macos-latest, 4.14.2)	33m 48s
install (8.19.0, Windows, windows-latest, 4.13.1, -async-proofs-j 1, git+https://github.com/ocaml...	1h 36m 10s
install (8.18.0, Ubuntu, ubuntu-latest, 4.09.1)	42m 34s
install (8.18.0, Windows, windows-latest, 4.13.1, -async-proofs-j 1, git+https://github.com/ocaml...	1h 39m 38s
install (8.20.0, Ubuntu, ubuntu-latest, 4.09.1)	39m 22s
	11h 32m 7s

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