README: replace cirrus badge with GHA badge

Author: jonasnick

README.md

@@ -1,7 +1,7 @@
 libsecp256k1
 ============
 
-[![Build Status](https://api.cirrus-ci.com/github/bitcoin-core/secp256k1.svg?branch=master)](https://cirrus-ci.com/github/bitcoin-core/secp256k1)
+[![Build Status](https://github.com/bitcoin-core/secp256k1/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/bitcoin-core/secp256k1/actions/workflows/ci.yml)
 ![Dependencies: None](https://img.shields.io/badge/dependencies-none-success)
 [![irc.libera.chat #secp256k1](https://img.shields.io/badge/irc.libera.chat-%23secp256k1-success)](https://web.libera.chat/#secp256k1)