Clear `$stdin` before showing prompt #419

Job	Run time
ruby-versions / Generate Ruby versions	2s
ruby-versions / Generate Ruby versions	2s
test (3.0)	1m 2s
test (3.0)	1m 5s
test (3.1)	1m 1s
test (3.1)	1m 2s
test (3.2)	1m 7s
test (3.2)	1m 2s
test (3.3)	1m 11s
test (3.3)	41s
test (3.4)	1m 3s
test (3.4)	1m 2s
test (debug)	1m 31s
test (debug)	1m 22s
test (head)	1m 9s
test (head)	1m 9s
	15m 31s

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