通过下面的代码实现了

using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Configuration;
using System;

namespace Cnblogs.Question
{
    class Program
    {
        static void Main(string[] args)
        {
            var host = new HostBuilder()
                .ConfigureAppConfiguration((context, configApp) =>
                {
                    configApp.AddEnvironmentVariables();
                }).Build();

            var conf = host.Services.GetRequiredService<IConfiguration>();
            Console.WriteLine(conf["conn"]);
        }
    }
}

需要安装的 nuget 包

<PackageReference Include="Microsoft.Extensions.Configuration" Version="2.2.0" />
<PackageReference Include="Microsoft.Extensions.Hosting" Version="2.2.0" />
<PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="2.2.0" />