haskell.compiler.ghcHEAD: increase Hydra timeout because Darwin builds were timing out

Building `haskell.compiler.ghcHEAD` on Hydra on Darwin seemed to cause
frequent timeouts.

Here's an example of a timeout: https://hydra.nixos.org/build/149762652

This commit raises the timeout time on Hydra for ghcHEAD. The other
GHC derivations have this set, so it makes sense to set this here
as well.

pkgs/development/compilers/ghc/head.nix

···
     homepage = "http://haskell.org/ghc";
     description = "The Glasgow Haskell Compiler";
     maintainers = with lib.maintainers; [ marcweber andres peti ];
+    timeout = 24 * 3600;
     inherit (ghc.meta) license platforms;
     # ghcHEAD times out on aarch64-linux on Hydra.
     hydraPlatforms = builtins.filter (p: p != "aarch64-linux") ghc.meta.platforms;