ci: `ubuntu-latest` is a better label for `20.04`

[azzamsa]

Pull Request

Related issue

Fixes #

What does this PR do?

• ...

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• [ ] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
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[curquiza]

Thanks for the PR, I close it following this comment: https://github.com/meilisearch/milli/issues/684#issuecomment-1305331395