Restore Mirage build in CI after upstream fixes

[aantron]

During #337 we discovered that many of the dependencies of dream-mirage are still being updated to mirage-crypto 1.0.0. This is fine, but it eventually prevented us from getting the Mirage build working again within a reasonable amount of time. This issue is a reminder to try it again once robur-coop/ocaml-letsencrypt#34 is released, and any other dependencies that are still waiting that we may discover subsequently.

[hannesm]

You'll need to wait for a release of the mirage tool -- with mirage/mirage#1559 being merged. I will ping you here once I think this should be a smooth sail. :)

[aantron]

Great, thank you!