pkgs/build-support/cabal: move current postFixup hook into installPhase to free up the postFixup hook for users to override

Builds tend to override pre and post hooks for each phase, so we should not use
them to perform regular build commands that cannot be omitted.

pkgs/build-support/cabal/default.nix

···
                 GHC_PACKAGE_PATH=$installedPkgConf ghc-pkg --global register $pkgConf --force
               fi
 
-              eval "$postInstall"
-            '';
-
-            postFixup = ''
               if test -f $out/nix-support/propagated-native-build-inputs; then
                 ln -s $out/nix-support/propagated-native-build-inputs $out/nix-support/propagated-user-env-packages
               fi
+
+              eval "$postInstall"
             '';
 
             # We inherit stdenv and ghc so that they can be used