os/mac/diagnostic: fix case of OpenCore check #31227

Job	Run time
syntax	34s
cask audit	3m 54s
formula audit	4m 46s
tap syntax	8m 9s
test default formula (Ubuntu 22.04)	1m 41s
tests (online)	5m 1s
test default formula (Ubuntu 20.04)	3m 0s
update-test (Ubuntu)	20s
tests (generic OS)	3m 4s
test default formula (macOS 13 x86_64)	3m 28s
update-test (macOS)	20s
tests (Ubuntu 24.04)	3m 5s
vendored gems	34s
test default formula (macOS 15 arm64)	2m 9s
tests (Ubuntu 22.04)	3m 4s
tests (Ubuntu 20.04)	4m 8s
tests (macOS 13 x86_64)	7m 13s
tests (macOS 15 arm64)	5m 28s
	59m 58s

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