Add top-level aliases for the runtime shell and its package.

This is like stdenv.shell{,Package}, but for the runtime system. The
majority of uses of stdenv.shell probably want this instead.

pkgs/top-level/all-packages.nix

@@ -5646,6 +5646,9 @@ with pkgs;
 
   ### SHELLS
 
+  runtimeShell = "${runtimeShellPackage}/bin/bash";
+  runtimeShellPackage = bash;
+
   bash = lowPrio (callPackage ../shells/bash/4.4.nix {
     texinfo = null;
     interactive = stdenv.isCygwin; # patch for cygwin requires readline support