Hack around wrong .mix directories for tests (backport #7493)

[mergify]

This is an automatic backport of pull request #7493 done by Mergify.
Cherry-pick of 2917995 has failed:

On branch mergify/bp/3.6/pr-7493
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	both modified:   cabal-install/cabal-install.cabal

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

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[Mikolaj]

The backport will fail, because test outputs are changed by other commits, but I will fixup the backport PR.