Use ghcup to install cabal head

[nlander]

This allows head to be the specified cabal version.

[andreasabel]

Thanks.

To evaluate the PR, please add a test to .github/workflows.
Once you are happy, please document the new feature in setup/README.

[nlander]

@andreasabel I have added a test and updated the README. I haven't run the test locally, because as far as I can tell it isn't part of the push hook. Please let me know if there is more I can do to help get this merged. Thanks!

[nlander]

@andreasabel I'm hoping these last two commits have fixed the test.

[nlander]

@andreasabel please take another look at this when you have a moment. Is there anything else you think this needs before merging?

[ulysses4ever]

@andreasabel indeed, cabal team would be delighted to have this feature merged.

@nlander thanks a lot for taking the time to prepare the patch!

[andreasabel]

Thanks, I merged it here.

The commit 77e8db1 needs to be transfered to haskell-actions/setup still.

[andreasabel]

I now opened this as a PR at the new home of the setup action:

• Use ghcup to install cabal head (haskell/actions#290) haskell-actions/setup#45