Fix not using builder nonce in token encoding

[brycx]

Previously, the nonce that the builder generated in new() was never being used, because a new nonce was generated in encode(). Thus, calling the nonce getter on the struct would never return the nonce actually used.

This PR changes the builder to generate a new nonce on each encode() call, such that retrieving the nonce via the struct, actually returns the nonce used to encrypt the most recent token.