Improve RUBYOPT's handling in tests #1715

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (2.7)	1m 49s
test (3.0)	1m 37s
test (3.1)	1m 50s
test (3.2)	2m 19s
test (3.3)	1m 46s
test (debug)	4m 58s
test (head)	1m 34s
	15m 56s

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