nix-shell: Fix bash completion

Nixpkgs's stdenv setup script sets the "nullglob" option, but doing so
breaks Bash completion on NixOS (when ‘programs.bash.enableCompletion’
is set) and on Ubuntu.  So clear that flag afterwards.  Of course,
this may break stdenv functions in subtle ways...

scripts/nix-build.in

@@ -210,6 +210,7 @@ foreach my $expr (@exprs) {
             'set +e; ' .
             'PS1="\n\[\033[1;32m\][nix-shell:\w]$\[\033[0m\] "; ' .
             'unset NIX_ENFORCE_PURITY; ' .
+            'shopt -u nullglob; ' .
             $envCommand);
         exec($ENV{NIX_BUILD_SHELL} // "bash", "--rcfile", $rcfile);
         die;