Configurable surrogate directory on Windows #2055

On first use, hyperlight writes hyperlight_surrogate_<hash>.exe to the directory of std::env::current_exe(), and sandbox creation fails if that directory is read-only (source). For a .NET app started with dotnet MyApp.dll, or tests run with dotnet test, that is the .NET install directory, by default C:\Program Files\dotnet. Make the directory configurable, for example with an environment variable.