Stable URLs for the ISO images

[fpletz]

To support netbooting NixOS from https://netboot.xyz/, we need a stable URL for the ISO images per channel.

Adding a redirect to the htaccess file

nixos-channel-scripts/mirror-nixos-branch.pl

Lines 213 to 215 in c3a9ba3

	write_file($htaccess,
	"Redirect /channels/$channelName $target\n" .
	"Redirect /releases/nixos/channels/$channelName $target\n");

should be should be enough.