Add GitLab CI badge (#9781)

* Add GitLab CI badge

* whitespace fixup

I managed to mess things up using the UI editor.

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Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: chreekat

README.md

@@ -5,6 +5,7 @@
 [![Documentation Status](http://readthedocs.org/projects/cabal/badge/?version=latest)](http://cabal.readthedocs.io/en/latest/?badge=latest)
 [![IRC chat](https://img.shields.io/badge/chat-via%20libera-brightgreen.svg)](https://web.libera.chat/#hackage)
 [![Matrix chat](https://img.shields.io/badge/chat-via%20matrix-brightgreen.svg)](https://matrix.to/#/#hackage:matrix.org)
+[![GitLab pipeline status](https://gitlab.haskell.org/haskell/cabal/badges/cabal-head/pipeline.svg?key_text=Release%20CI%20Early%20Warning&key_width=150)](https://gitlab.haskell.org/haskell/cabal/-/commits/cabal-head)
 
 <img src="https://www.haskell.org/cabal/images/Cabal-light.png" align="right">