mirrors.json file is expired

* Actions: view https://hackage.haskell.org/mirrors.json
* Expected: `signed.expires` field will be in the future
* Actual: it's in the past, specifically yesterday

This is resulting in bug reports from users of Stack trying to initialize a new `hackage-security`-based package index:

* https://github.com/commercialhaskell/stack/issues/3731
* https://github.com/commercialhaskell/stack/issues/4927

I'm currently considering working around this by disabling the expiry checks by default in Stack's usage of `hackage-security`.

Start by viewing https://hackage.haskell.org/mirrors.json and checking the signed.expires value against the current date. Read the linked Stack issues for the reported initialization failures; done means the published expiry is in the future and new hackage-security-based package indexes can initialize without this failure.

Written by the indexing model from the issue text.