cmd/setup-ruby: fix Portable Ruby install failing #28055

Job	Run time
syntax	52s
vendored gems	48s
cask audit	3m 19s
docker	4m 58s
formula audit	6m 38s
tap syntax	5m 14s
test default formula (Ubuntu 22.04)	4m 20s
test default formula (Ubuntu 20.04)	5m 42s
tests (online)	8m 54s
update-test (Ubuntu 22.04)	1m 4s
test default formula (macOS 13 x86_64)	2m 37s
tests (generic OS)	6m 13s
test default formula (macOS 14 arm64)	1m 23s
update-test (macOS 13)	53s
tests (Ubuntu 22.04)	6m 13s
tests (Ubuntu 20.04)	7m 4s
tests (macOS 13 x86_64)	5m 49s
tests (macOS 14 arm64)	4m 52s
	1h 16m 53s

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