[CI] [opam] Work around MacOS / opam / brew issue #762

Job	Run time
docker-master	1h 30m 7s
publish-standalone-dry-run	14s
test-standalone (master, dev, default)	1m 27s
build-wasm-of-ocaml (master, 4.14.1)	1s
build-js-of-ocaml (master, 4.11.1)	12m 47s
validate-docker-master	24m 31s
generated-files (master)	34s
standalone-haskell (master)	45m 29s
test-amd64	2h 17m 27s
deploy-js-wasm-of-ocaml	0s
docker-check-all	6s
	5h 12m 43s

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