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.

Author: nfischer

README.md

@@ -1,6 +1,6 @@
 # shelljs-plugin-open
 
-[![Build Status](https://img.shields.io/endpoint.svg?url=https%3A%2F%2Factions-badge.atrox.dev%2Fshelljs%2Fplugin-open%2Fbadge%3Fref%3Dmain&style=flat-square)](https://actions-badge.atrox.dev/shelljs/plugin-open/goto?ref=main)
+[![GitHub Actions](https://img.shields.io/github/actions/workflow/status/shelljs/plugin-open/main.yml?style=flat-square&logo=github)](https://github.com/shelljs/plugin-open/actions/workflows/main.yml)
 [![npm](https://img.shields.io/npm/v/shelljs-plugin-open.svg?style=flat-square)](https://www.npmjs.com/package/shelljs-plugin-open)
 [![shelljs-plugin](https://img.shields.io/badge/shelljs-plugin-brightgreen.svg?style=flat-square)](https://github.com/shelljs/shelljs/wiki/Using-ShellJS-Plugins)