Clear `$stdin` before showing prompt #1749

Job	Run time
ruby-versions / Generate Ruby versions	3s
ruby-versions / Generate Ruby versions	3s
test (2.7)	2m 2s
test (2.7)	2m 5s
test (3.0)	2m 2s
test (3.0)	1m 38s
test (3.1)	1m 41s
test (3.1)	1m 41s
test (3.2)	1m 56s
test (3.2)	1m 34s
test (3.3)	1m 48s
test (3.3)	1m 48s
test (3.4)	1m 35s
test (3.4)	1m 35s
test (debug)	5m 4s
test (debug)	5m 4s
test (head)	1m 40s
test (head)	1m 40s
	34m 59s

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