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.

Changed files
+3
pkgs
top-level
+3
pkgs/top-level/all-packages.nix
···
### SHELLS
bash = lowPrio (callPackage ../shells/bash/4.4.nix {
texinfo = null;
interactive = stdenv.isCygwin; # patch for cygwin requires readline support
···
### 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