Backport #10654: CI: remove cabal-head before updating it (fix #10537)

[mergify]

This PR is #10540 rebased.

It should fix #10537 and its duplicate #10652

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E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• [n/a] Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

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This is an automatic backport of pull request #10654 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 9949014 has failed:

On branch mergify/bp/3.12/pr-10654
Your branch is up to date with 'origin/3.12'.

You are currently cherry-picking commit 994901461.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/validate.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[geekosaur]

Mmm, this isn't going to work; I'll need to cherry-pick the token fix into it and kill the other backport.
/cc: @Kleidukos