fix(scripts): confirm script should default to Y

[hawkw]

The confirm helper function used in our build scripts prints "[Y/n]", which suggests that "yes" is the default option. However, if you press enter, it just says "invalid input" and retries, which is not what it claims to do. This commit fixes that.

For example, a test script like:

#!/usr/bin/env bash

source scripts/_util.sh

if confirm "test"; then
    echo "ok you confirmed"
else
    echo "you didn't confirm"
fi

will now default to "yes" if you just press enter:

$ ./test/sh
test [Y/n]
ok you confirmed