Export shell packages from stdenvLinux

Now that we have a way to alter /bin/sh in chroots on non-NixOS Linux
platforms, it may be useful to have access to stdenv.shell package and to
static bash contained in bootstrap tools. So make them accessible via stdenv
attribute set.

pkgs/stdenv/linux/default.nix

@@ -284,7 +284,8 @@ rec {
 
     extraAttrs = {
       inherit (stdenvLinuxBoot3Pkgs) glibc;
-      inherit platform;
+      inherit platform bootstrapTools;
+      shellPackage = stdenvLinuxBoot4Pkgs.bash; 
     };
 
     overrides = pkgs: {