You can easily access it in ConfigureServices, just persist it to a property during Startup method which is called first and gets it passed in, then you can access the property from ConfigureServices.
public Startup(IWebHostEnvironment env, IApplicationEnvironment appEnv)
{
...your code here...
CurrentEnvironment = env;
}
private IWebHostEnvironment CurrentEnvironment{ get; set; }
public void ConfigureServices(IServiceCollection services)
{
string envName = CurrentEnvironment.EnvironmentName;
... your code here...
}