Add a ci test for cabal-install self-upgrade policy

In 6c2eba7fca96872b6c8e3bde711feadf3491427f we clarified our `cabal-install` support policy with respect to self-upgrade: upgrading to the latest version with `cabal install cabal-install` must work with all `cabal-install` versions released in the last three years. As discussed in #4250, we should also have a CI check that this guarantee actually holds. Testing that `cabal install Cabal/ cabal-install/` succeeds with `cabal-install` 1.18 should be enough.