[CI] Use badge in gist for rules number in README

Since the following PR, the README badge in capa is not updated anymore
via commits anymore, but in a gist:
mandiant/capa-rules#882

README.md

@@ -2,7 +2,7 @@
 
 [![PyPI - Python Version](https://img.shields.io/pypi/pyversions/flare-capa)](https://pypi.org/project/flare-capa)
 [![Last release](https://img.shields.io/github/v/release/mandiant/capa)](https://github.com/mandiant/capa/releases)
-[![Number of rules](https://img.shields.io/badge/rules-641-blue.svg)](https://github.com/mandiant/capa-rules)
+[![Number of rules](https://gist.githubusercontent.com/capa-bot/6d7960e911f48b3b74916df8988cf0f3/raw/rules_badge.svg)](rules)
 [![CI status](https://github.com/mandiant/capa/workflows/CI/badge.svg)](https://github.com/mandiant/capa/actions?query=workflow%3ACI+event%3Apush+branch%3Amaster)
 [![Downloads](https://img.shields.io/github/downloads/mandiant/capa/total)](https://github.com/mandiant/capa/releases)
 [![License](https://img.shields.io/badge/license-Apache--2.0-green.svg)](LICENSE.txt)