ghc: make sure the HEAD version is built with a deterministic version number

pkgs/development/compilers/ghc/head.nix

···
 
   postUnpack = ''
     pushd ghc-${builtins.substring 0 7 rev}
+    echo ${version} >VERSION
+    echo ${rev} >GIT_COMMIT_ID
     patchShebangs .
     ./boot
     popd