Add CI shield for opam package to README #4674

Job	Run time
macOS 12 (x86_64)	3h 39m 2s
macOS 14 (arm64)	1h 56m 15s
combine-standalone	15s
publish-standalone	22s
publish-standalone (-x86_64)	17s
test-standalone (macos-12)	14s
publish-standalone (-arm64)	22s
test-standalone (macos-13)	18s
test-standalone (macos-14)	14s
test-standalone (macos-latest)	10s
test-standalone (-x86_64, macos-12)	15s
test-standalone (-x86_64, macos-13)	24s
test-standalone (-x86_64, macos-14)	12s
test-standalone (-x86_64, macos-latest)	12s
test-standalone (-arm64, macos-14)	11s
test-standalone (-arm64, macos-latest)	12s
macos-check-all	1s
	5h 38m 56s

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