Update should support trusting ssh keys

[jsoref]

This PR typelead/eta#989 ran into this error:

https://app.circleci.com/pipelines/github/typelead/eta/1/workflows/0353209f-42ce-469c-a4f5-57a09f7fa2e6/jobs/2762

Downloading the latest package lists from:
- hackage.haskell.org
- git@github.com/typelead/etlas-index
The authenticity of host 'github.com (140.82.114.4)' can't be established.
ECDSA key fingerprint is SHA256:p2QAMXNIC1TJYWeIOttrVc98/R1BUFWu3/LiyKgUfQM.
Are you sure you want to continue connecting (yes/no)? To revert to previous state run:
    etlas update --index-state='2022-07-31T16:27:39Z'

Too long with no output (exceeded 10m0s): context deadline exceeded

The code appears to be here:

etlas/etlas/Distribution/Client/Update.hs

Line 60 in bbd7c55

update :: Verbosity -> UpdateFlags -> RepoContext -> FilePath -> Bool -> IO ()

Perhaps the client should just add some magic config to the .ssh/config environment, but it's traditional to let the caller ask the callee to manage such ssh details for it...