Fix index-state syntax

The second index-state stanza completely ovverides the first, resetting hackage index state to HEAD.
See haskell/cabal#8568

Author: andreabedini

cabal.project

@@ -10,8 +10,9 @@ repository cardano-haskell-packages
     c00aae8461a256275598500ea0e187588c35a5d5d7454fb57eac18d9edb86a56
     d4a35cd3121aa00d18544bb0ac01c3e1691d618f462c46129271bccf39f7e8ee
 
-index-state: 2022-11-06T00:00:00Z
-index-state: cardano-haskell-packages 2022-11-15T17:30:00Z
+index-state:
+  , hackage.haskell.org 2022-11-06T00:00:00Z
+  , cardano-haskell-packages 2022-11-15T17:30:00Z
 
 packages: ./io-sim
           ./io-classes