Fix pin operator compile warnings

[mitchellhenke]

There's a couple compile warnings on Elixir 1.20 for the pin operator, and this PR fixes them 😀

     warning: the variable "key_length" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 275 │     <<_::size(key_length)-binary, _::4, offset::4>> = hmac
     │               ~
     │
     └─ lib/nimble_totp.ex:275:15: NimbleTOTP.hmac_truncate/1

     warning: the variable "offset" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 276 │     <<_::size(offset)-binary, p::4-binary, _::binary>> = hmac
     │               ~
     │
     └─ lib/nimble_totp.ex:276:15: NimbleTOTP.hmac_truncate/1