If there was a known attack it would obviously be a non-starter. But why take the risk that someone will come up with something comparable - in the broadest sense - to say the differential attack on MD5 that allows exploiting known bits in the salt if there is no need to?
For password hashing purposes, salt doesn't need to be uniformly random, the only requirement for salt is to be unique and unpredictable to the attacker (see http://crypto.stackexchange.com/questions/6119/hashing-passw...). Most password hashing functions use a cryptographic hash on salt.
This particular function, scrypt, uses one-round PBKDF2-HMAC-SHA256 to mix password and salt:
https://github.com/golang/crypto/blob/master/scrypt/scrypt.g...
PBKDF2 feeds salt, basically, into SHA256:
https://github.com/golang/crypto/blob/master/pbkdf2/pbkdf2.g...