x.py setup: Fix handling of wrong interactive input
x.py setup: Fix handling of wrong interactive input

We need a fresh input buffer each time, or we reuse the previous
data (since `read_line` appends).

Signed-off-by: Ian Jackson <d68d4287c1430ca128ad85033356c9d259238c74@chiark.greenend.org.uk>

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust