mkshell: improve mergeInputs

mergeInputs is now simply defined in terms of `concatLists` and
`catAttrs` instead of a more complicated `foldr`.

Note that the order of PATH has also changed. For example running the
following with nix-shell:

let
pkgs = import <nixpkgs> {};

shell1 = pkgs.mkShell {
buildInputs = [ pkgs.htop ];
};

shell2 = pkgs.mkShell {
buildInputs = [ pkgs.hello ];
};

shell3 = pkgs.mkShell {
inputsFrom = [ shell1 shell2 ];
buildInputs = [ pkgs.tree ];
};

in shell3

Results in the following PATH:

$ echo $PATH
...
/nix/store/yifq4bikf7m07160bpia7z48ciqddbfi-tree-1.8.0/bin:
/nix/store/vhxqk81234ivqw1a7j200a1c69k8mywi-htop-2.2.0/bin:
/nix/store/n9vm3m58y1n3rg3mlll17wanc9hln58k-hello-2.10/bin
...

Previously the order was:

/nix/store/n9vm3m58y1n3rg3mlll17wanc9hln58k-hello-2.10/bin
/nix/store/vhxqk81234ivqw1a7j200a1c69k8mywi-htop-2.2.0/bin:
/nix/store/yifq4bikf7m07160bpia7z48ciqddbfi-tree-1.8.0/bin:

I think the new order makes more sense because it allows to override
the PATH in the outermost mkShell.

pkgs/build-support/mkshell/default.nix

···
   ...
 }@attrs:
 let
-  mergeInputs = name:
-    let
-      op = item: sum: sum ++ item."${name}" or [];
-      nul = [];
-      list = [attrs] ++ inputsFrom;
-    in
-      lib.foldr op nul list;
+  mergeInputs = name: lib.concatLists (lib.catAttrs name
+    ([attrs] ++ inputsFrom));
 
   rest = builtins.removeAttrs attrs [
     "inputsFrom"