README.md: update GitHub workflow badge

The GitHub workflow badge fails: badges/shields#8671
So update the GitHub workflow badge to the new location.

Signed-off-by: Benjamin Drung <bdrung@posteo.de>

README.md

@@ -1,4 +1,4 @@
-[![CI](https://img.shields.io/github/workflow/status/bdrung/bdebstrap/CI)](https://github.com/bdrung/bdebstrap/actions/workflows/ci.yaml)
+[![CI](https://img.shields.io/github/actions/workflow/status/bdrung/bdebstrap/ci.yaml)](https://github.com/bdrung/bdebstrap/actions/workflows/ci.yaml)
 [![Codecov](https://img.shields.io/codecov/c/github/bdrung/bdebstrap)](https://codecov.io/gh/bdrung/bdebstrap)
 [![Code Style: black](https://img.shields.io/badge/code%20style-black-black)](https://github.com/psf/black)
 [![License: ISC](https://img.shields.io/badge/license-ISC-blue)](LICENSE)