Fix make_shareable shim in harness-common.rb #1501

Job	Run time
test (ruby)	54s
test (head)	47s
benchmark-default (truffleruby)	15m 59s
test (truffleruby)	59s
benchmark-default (ruby)	10m 50s
benchmark-default (head)	11m 11s
benchmark-ractor (head)	12m 30s
benchmark-run-once	1m 40s
benchmark-graph	1m 4s
benchmark-ractor-only (ruby)	52s
benchmark-ractor (ruby)	12m 46s
benchmark-ractor-only (head)	51s
all-tests	2s
	1h 10m 25s

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