docs: change GitHub Actions README badge

This changes the README to use a standard shields.io badge for GitHub
Actions. The custom badge
(https://github.com/Atrox/github-actions-badge) was cool, but the
atrox.dev link doesn't work reliably. I noticed that shields has support
for this now, so there's no reason to use a custom badge.

One notably difference is that this badge says the build is passing even
if there's a job still in progress.

README.md

@@ -1,6 +1,6 @@
 # ShellJS - Unix shell commands for Node.js
 
-[![Build Status](https://img.shields.io/endpoint.svg?url=https%3A%2F%2Factions-badge.atrox.dev%2Fshelljs%2Fshelljs%2Fbadge%3Fref%3Dmaster&style=flat-square)](https://actions-badge.atrox.dev/shelljs/shelljs/goto?ref=master)
+[![GitHub Actions][https://img.shields.io/github/actions/workflow/status/shelljs/shelljs/main.yml?style=flat-square&logo=github]][https://github.com/shelljs/shelljs/actions/workflows/main.yml]
 [![Codecov](https://img.shields.io/codecov/c/github/shelljs/shelljs/master.svg?style=flat-square&label=coverage)](https://codecov.io/gh/shelljs/shelljs)
 [![npm version](https://img.shields.io/npm/v/shelljs.svg?style=flat-square)](https://www.npmjs.com/package/shelljs)
 [![npm downloads](https://img.shields.io/npm/dm/shelljs.svg?style=flat-square)](https://www.npmjs.com/package/shelljs)