readme: build badge for Github actions, not Azure pipeline (#3825)

Author: bergey

README.md

@@ -7,7 +7,7 @@
   </p>
 
   <p>
-    <a href="https://dev.azure.com/rustwasm/wasm-bindgen/_build/latest?definitionId=1&branchName=master"><img src="https://img.shields.io/azure-devops/build/rustwasm/wasm-bindgen/1.svg?style=flat-square" alt="Build Status" /></a>
+    <a href="https://github.com/rustwasm/wasm-bindgen/actions/workflows/main.yml?query=branch%3Amain"><img src="https://github.com/rustwasm/wasm-bindgen/actions/workflows/main.yml/badge.svg?branch=main" alt="Build Status" /></a>
     <a href="https://crates.io/crates/wasm-bindgen"><img src="https://img.shields.io/crates/v/wasm-bindgen.svg?style=flat-square" alt="Crates.io version" /></a>
     <a href="https://crates.io/crates/wasm-bindgen"><img src="https://img.shields.io/crates/d/wasm-bindgen.svg?style=flat-square" alt="Download" /></a>
     <a href="https://docs.rs/wasm-bindgen"><img src="https://img.shields.io/badge/docs-latest-blue.svg?style=flat-square" alt="docs.rs docs" /></a>