Fix Azure Pipelines build badge in README (#78134)

It wasn't updated after the dnceng-public move.

README.md

 # .NET Runtime
 
-[![Build Status](https://dnceng.visualstudio.com/public/_apis/build/status/dotnet/runtime/runtime?branchName=main)](https://dnceng.visualstudio.com/public/_build/latest?definitionId=686&branchName=main)
+[![Build Status](https://dev.azure.com/dnceng-public/public/_apis/build/status/dotnet/runtime/runtime?branchName=main)](https://dev.azure.com/dnceng-public/public/_build/latest?definitionId=129&branchName=main)
 [![Help Wanted](https://img.shields.io/github/issues/dotnet/runtime/help%20wanted?style=flat-square&color=%232EA043&label=help%20wanted)](https://github.com/dotnet/runtime/labels/help%20wanted)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/dotnet/runtime)
 [![Discord](https://img.shields.io/discord/732297728826277939?style=flat-square&label=Discord&logo=discord&logoColor=white&color=7289DA)](https://aka.ms/dotnet-discord)