Reintroduce `cabal update` as ghc user

Previously 26975eeb attempted to
optimise the build by unifying the `cabal update` invocations. However,
this optimisation is in fact wrong. We must run `cabal update` twice:
once as root to build the bootstrap tools and again as the ghc user
(which will be used by downstream GHC builds).

images/alpine.dhall

@@ -85,6 +85,7 @@ let
       ]
     # [CF.Statement.User "ghc"]
     # CF.workdir "/home/ghc/"
+    # CF.run "update cabal index" [ "$CABAL update"]
 
 let
   images: List Image.Type =

images/centos.dhall

@@ -91,6 +91,7 @@ let
       ]
     # [CF.Statement.User "ghc"]
     # CF.workdir "/home/ghc/"
+    # CF.run "update cabal index" [ "$CABAL update"]
 
 let
   CentosImage =

images/debian.dhall

@@ -103,6 +103,7 @@ let
       ]
     # [CF.Statement.User "ghc"]
     # CF.workdir "/home/ghc/"
+    # CF.run "update cabal index" [ "$CABAL update"]
 
 -- Install stack for testing hadrian
 let

images/fedora.dhall

@@ -87,6 +87,7 @@ let
       ]
     # [CF.Statement.User "ghc"]
     # CF.workdir "/home/ghc/"
+    # CF.run "update cabal index" [ "$CABAL update"]
 
 let
   FedoraImage =