Fix build with older Clang

[pavelzw]

See #2397

  /tmp/micromamba-20230627-77271-10613rh/mamba-micromamba-1.4.5/libmamba/src/solv-cpp/pool.cpp:135:27: error: chosen constructor is explicit in copy-initialization
          std::stringstream msg = {};
                            ^     ~~
  /Library/Developer/CommandLineTools/SDKs/MacOSX11.sdk/usr/include/c++/v1/sstream:884:14: note: explicit constructor declared here
      explicit basic_stringstream(ios_base::openmode __wch = ios_base::in | ios_base::out);
               ^
  1 error generated.
  make[2]: *** [libmamba/CMakeFiles/libmamba.dir/src/solv-cpp/pool.cpp.o] Error 1

Occurred in Homebrew/homebrew-core#135110

[jonashaag]

@pavelzw is the build failure related?

[pavelzw]

is the build failure related

No, it fails during a VCPKG step before.