[CI] [opam] [MacOS] fix kludge for pkg-conf #764

Job	Run time
docker-master	1h 31m 48s
publish-standalone-dry-run	14s
test-standalone (master, dev, default)	1m 38s
build-js-of-ocaml (master, 4.11.1)	12m 18s
validate-docker-master	21m 55s
build-wasm-of-ocaml (master, 4.14.1)	17m 21s
generated-files (master)	29s
standalone-haskell (master)	46m 3s
test-amd64	2h 17m 50s
deploy-js-wasm-of-ocaml	49s
docker-check-all	4s
	5h 30m 29s

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