dockerTools: mark store-path-to-layer.sh as executable

bcf54ce5bb introduced a treewide change to
use ${stdenv.shell} where-ever possible. However, this broke a script
used by dockerTools, store-path-to-layer.sh, as it did not preserve the
+x mode bit. This meant the file got put into the store as mode 0444,
resulting in a build-time error later on that looked like:

    xargs: /nix/store/jixivxhh3c8sncp9xlkc4ls3y5f2mmxh-store-path-to-layer.sh: Permission denied

However, in a twist of fate, bcf54ce5bb
not only introduced this regression but, in this particular instance,
didn't even fix the original bug: the store-path-to-layer.sh script
*still* uses /bin/sh as its shebang line, rather than an absolute path
to stdenv. (Fixing this can be done in a separate commit.)

Signed-off-by: Austin Seipp <aseipp@pobox.com>

pkgs/build-support/docker/default.nix

@@ -283,6 +283,7 @@ rec {
     let
       storePathToLayer = substituteAll
       { inherit (stdenv) shell;
+        isExecutable = true;
         src = ./store-path-to-layer.sh;
       };
     in