Fix make_shareable shim in harness-common.rb #1499

Job	Run time
benchmark-default (head)	10m 42s
benchmark-default (truffleruby)	15m 34s
test (truffleruby)	57s
benchmark-default (ruby)	10m 54s
test (head)	1m 1s
test (ruby)	55s
benchmark-ractor (ruby)	12m 47s
benchmark-ractor (head)	12m 31s
benchmark-run-once	1m 34s
benchmark-graph	55s
benchmark-ractor (truffleruby)	17s
benchmark-ractor-only (ruby)	53s
benchmark-ractor-only (head)	46s
benchmark-ractor-only (truffleruby)	11s
all-tests	4s
	1h 10m 1s

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