Follow the channel server's redirect and offer the stable releases

channels.nixos.org answers <branch>/git-revision with a redirect, which
router-upgrade check did not follow: on a real router it saw an empty
answer. The test channel is now an HTTP server that redirects the same way.

The page labelled the running release "stable", but nixos-unstable already
calls itself the next release (26.11 does not exist as a branch yet). It
offers none, unstable and the stable releases listed in the code (26.05).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

modules/settings/upgrade.nix

@@ -84,7 +84,8 @@ let
 
       newest_rev() {
         local rev
-        rev=$(curl -fsS --max-time 30 "${up.channelsUrl}/$branch/git-revision") \
+        # (channels.nixos.org answers with a redirect to releases.nixos.org.)
+        rev=$(curl -fsSL --max-time 30 "${up.channelsUrl}/$branch/git-revision") \
           || die "cannot read ${up.channelsUrl}/$branch/git-revision (no internet, or no channel of that name)"
         [[ $rev =~ ^[0-9a-f]{40}$ ]] || die "unexpected answer for $branch: ''${rev:0:60}"
         echo "$rev"