mkShell: compose shellHooks

Running the following expression with nix-shell:

  let
    pkgs = import <nixpkgs> {};

    shell1 = pkgs.mkShell {
      shellHook = ''
        echo shell1
      '';
    };

    shell2 = pkgs.mkShell {
      shellHook = ''
        echo shell2
      '';
    };

    shell3 = pkgs.mkShell {
      inputsFrom = [ shell1 shell2 ];
      shellHook = ''
        echo shell3
      '';
    };
  in shell3

Will now results in:
shell2
shell1
shell3

Note that packages in the front of inputsFrom have precedence over
packages in the back. The outermost mkShell has precedence over all.

pkgs/build-support/mkshell/default.nix

@@ -25,6 +25,7 @@ let
     "nativeBuildInputs"
     "propagatedBuildInputs"
     "propagatedNativeBuildInputs"
+    "shellHook"
   ];
 in
 
@@ -37,6 +38,9 @@ stdenv.mkDerivation ({
   propagatedBuildInputs = mergeInputs "propagatedBuildInputs";
   propagatedNativeBuildInputs = mergeInputs "propagatedNativeBuildInputs";
 
+  shellHook = lib.concatStringsSep "\n" (lib.catAttrs "shellHook"
+    (lib.reverseList inputsFrom ++ [attrs]));
+
   nobuildPhase = ''
     echo
     echo "This derivation is not meant to be built, aborting";