fix compiler warning in elixir 1.20

[slashmili]

Fixes compiler warning in the upcoming elixir:

elixir --version
Erlang/OTP 28 [erts-16.3] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1]

Elixir 1.20.0-rc.3 (2db87eb) (compiled with Erlang/OTP 28)

mix test
     warning: the variable "size" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     \x{2502}
 371 \x{2502}     <<i::unsigned-size(size)-unit(8)>> = :crypto.strong_rand_bytes(size)
     \x{2502}                        ~
     \x{2502}
     \x{2514}\x{2500} lib/x509/certificate.ex:371:24: X509.Certificate.random_serial/1

[voltone]

Thanks! Small change request to not break compatibility with older Elixir versions

[voltone]

It seems the use of the pin operator recommended in Elixir 1.20 is not supported by Elixir <=1.14. We can sidestep the issue altogether by using :binary.decode_unsigned/1.