Passing an index-state timestamp to cabal update currently makes it
write a message saying the index has been updated to the *previous*
timestamp.

    $ cabal update hackage.haskell.org,2016-09-24T17:47:48Z
    Downloading the latest package list from hackage.haskell.org
    Package list of hackage.haskell.org is up to date at index-state 2022-01-27T12:59:23Z

This is because of a confusion between downloading the index and setting
the index state. These are independent actions.

This patch makes separates the two messages (updating the index and
updating the index-state timestamp) and makes sure the right timestamp
is printed.