Fix make_shareable shim in harness-common.rb #1492

Job	Run time
benchmark-default (head)	10m 40s
test (head)	11s
test (ruby)	10s
test (truffleruby)	30s
benchmark-default (ruby)	11m 2s
benchmark-graph	1m 8s
benchmark-ractor (ruby)	13m 20s
benchmark-default (truffleruby)	14m 39s
benchmark-ractor (head)	12m 24s
benchmark-run-once	1m 33s
benchmark-ractor (truffleruby)	19s
benchmark-ractor-only (head)	48s
benchmark-ractor-only (truffleruby)	13s
benchmark-ractor-only (ruby)	56s
all-tests	2s
	1h 7m 55s

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