README: use the correct badge (#3758)

1. The github README is probably the first encounter for people looking for Oscar (it is the first hit on various search engines).
2. I think it gives a very bad first impression for non-developers, if they navigate to the  and see right at the top a big scary red "tests failing". What it is supposed to tell someone? Don't use it? Use it and expect wrong results?
3. A "tests failing" is always a CI fluke or some other non-actionable things.
4. A green "tests passing" gives a fuzzy warm feeling.

To summarize:
- red bagde = scary and useless, shall I use Oscar?
- green badge = nice, I want to use Oscar!

My actual proposal of a static "tests passing" badge (without any connection to the actual status of CI) might be a bit controversial, so here is a less controversial one. Just show the badge for the push action on the master branch.

README.md

@@ -158,7 +158,7 @@ Forschungsgemeinschaft DFG within the
 [docs-stable-img]: https://img.shields.io/badge/docs-stable-blue.svg
 [docs-stable-url]: https://docs.oscar-system.org/stable/
 
-[ga-img]: https://github.com/oscar-system/Oscar.jl/workflows/Run%20tests/badge.svg
+[ga-img]: https://github.com/oscar-system/Oscar.jl/actions/workflows/CI.yml/badge.svg?branch=master&event=push
 [ga-url]: https://github.com/oscar-system/Oscar.jl/actions?query=workflow%3A%22Run+tests%22
 
 [codecov-img]: https://codecov.io/gh/oscar-system/Oscar.jl/branch/master/graph/badge.svg?branch=master