Change String not to be reassigned #1289

Job	Run time
test-memory (head)	36s
check-misc (head)	11s
test-ruby (head, master)	5m 33s
steep-check (head)	26s
test-ruby (3.0, master)	8m 59s
test-windows (head)	3m 43s
ruby-versions / Generate Ruby versions	2s
test (2.5)	25s
test (2.6)	26s
test (2.7)	27s
test (3.0)	13s
test (3.1)	13s
test (3.2)	14s
test (3.3)	16s
test (head)	14s
	21m 58s

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