Clear `$stdin` before showing prompt #418

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (3.0)	1m 0s
test (3.1)	1m 1s
test (3.2)	1m 4s
test (3.3)	1m 9s
test (3.4)	59s
test (debug)	1m 19s
test (head)	1m 3s
	7m 38s

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