my_ip_cstr获取本机ip为127.0.0.1

**Describe the bug (描述bug)**

butil::my_ip_cstr() 获取本机的ip为127.0.0.1

**To Reproduce (复现方法)**

**Expected behavior (期望行为)**
期望获取到真实的ip

**Versions (各种版本)**
OS:
Compiler:
brpc:
protobuf:

**Additional context/screenshots (更多上下文/截图)**

Start at the butil::my_ip_cstr() entry point and reproduce the report in an environment where it returns 127.0.0.1. Check how the function selects the local address and document the relevant OS, compiler, brpc, and protobuf versions. Done means it returns the expected real IP without regressing other environments.

Written by the indexing model from the issue text.