How to ensure that r and N are relatively prime

[realTaki]

line 106 func EncryptAndNonce(pubKey *PublicKey, plainText []byte) ([]byte, *big.Int, error)
line 107 r, err := rand.Int(rand.Reader, pubKey.N)
r just a random int,
How to ensure that r and N are relatively prime , gcd(r,N)=1?