`time_timestamp` is affected by the year 2038 problem

[Robyt3]

It currently returns the unix timestamp as an int, which will overflow in the year 2038:

https://github.com/ddnet/ddnet/blob/9336b4cb37fdbcaa647bc1670a42ab0e7527a1e6/src/base/system.cpp#L2616-L2619

See https://en.wikipedia.org/wiki/Year_2038_problem