Fix compiler warnings in fuzzer.cpp

[StanFromIreland]

I was seeing:

/home/stan/dev/library-fuzzers/fuzzer.cpp:20:20: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
   20 |     char argv0Copy[argv0.size()+1];
      |                    ^~~~~~~~~~~~~~
/home/stan/dev/library-fuzzers/fuzzer.cpp:20:26: note: non-constexpr function 'size' cannot be used in a constant expression
   20 |     char argv0Copy[argv0.size()+1];
      |                          ^

We can use std::vector instead.