Protect pushes to NixOS/nixpkgs channel branches with a special commit status.

[grahamc]

IT is possible for admins to push and merge branches in to the protected channel branches. This is a mistake I made.

To prevent this, we should have the channel scripts set a special status on the commit and then require pushes to those branches to have that special status.

[zimbatm]

This would be so much easier if we had a separate repo ^_^