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.

(cherry picked from commit 76ef802)

pkgs/build-support/mkshell/default.nix

@@ -20,6 +20,7 @@ let
     "nativeBuildInputs"
     "propagatedBuildInputs"
     "propagatedNativeBuildInputs"
+    "shellHook"
   ];
 in
 
@@ -32,6 +33,9 @@ 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";