Switch build status badge to GH action (#2005)

README.md

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-# Temporal Java SDK  [![Build status](https://badge.buildkite.com/663f6d1be81be6700c28c242b35905f20b68c4fda7b2c7c4e3.svg?branch=master)](https://buildkite.com/temporal/java-sdk-public) [![Coverage Status](https://coveralls.io/repos/github/temporalio/sdk-java/badge.svg?branch=master)](https://coveralls.io/github/temporalio/sdk-java?branch=master)
+# Temporal Java SDK  [![Build status](https://github.com/temporalio/sdk-java/actions/workflows/ci.yml/badge.svg?event=push)](https://github.com/temporalio/sdk-java/actions/workflows/ci.yml) [![Coverage Status](https://coveralls.io/repos/github/temporalio/sdk-java/badge.svg?branch=master)](https://coveralls.io/github/temporalio/sdk-java?branch=master)
 
 [Temporal](https://github.com/temporalio/temporal) is a Workflow-as-Code platform for building and operating
 resilient applications using developer-friendly primitives, instead of constantly fighting your infrastructure.