head-node: Fix unattended upgrades by specifying the right Cuirass branch.

Previously unattended upgrades would fail because the cached ‘master’
branch had a commit older than the deployed ‘main’ branch.

* head-node.scm (unattended-upgrade-configuration)[channels]: Use branch
‘main’ of ‘cuirass’ instead of ‘master’.

head-node.scm

@@ -491,6 +491,7 @@ export CUIRASS_LOGGING_LEVEL=${CUIRASS_LOGGING_LEVEL:-debug}\n"
                             #~(cons* (channel
                                       (name 'cuirass)
                                       (url "https://git.savannah.gnu.org/git/guix/guix-cuirass.git")
+                                      (branch "main")
                                       (introduction
                                        (make-channel-introduction
                                         "c75620777c33273fcd14261660288ec1b2dc8123"