readme: update build job badge url
## [Website](https://tikv.org) | [Documentation](https://tikv.org/docs/latest/concepts/overview/) | [Community Chat](https://tikv.org/chat)
-[![Build Status](https://internal.pingcap.net/idc-jenkins/job/build_tikv_master/badge/icon)](https://internal.pingcap.net/idc-jenkins/job/build_tikv_master/)
+[![Build Status](https://internal.pingcap.net/idc-jenkins/buildStatus/icon?job=build_tikv_multi_branch%2Fmaster)](https://internal.pingcap.net/idc-jenkins/job/build_tikv_multi_branch/)
[![Coverage Status](https://coveralls.io/repos/github/tikv/tikv/badge.svg?branch=master)](https://coveralls.io/github/tikv/tikv?branch=master)
[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/2574/badge)](https://bestpractices.coreinfrastructure.org/projects/2574)
