Align the China region to the official cn profile

The region value is cn, the China files live under default/regions/cn/, and
omarchy-apply-pacman renders pacman for every caller: the .cn twins of the
channel servers first, then .org, USTC and ArchLinuxCN. Switching to China
installs and trusts the ArchLinuxCN keyring. A migration renames china to cn
and repoints the drop-in links.

migrations/1791387144.sh

@@ -0,0 +1,11 @@
+echo "Rename the China region to cn and follow its moved profile"
+
+[[ $(cat /etc/omarchy/region 2>/dev/null) == "china" ]] || exit 0
+
+echo cn | sudo tee /etc/omarchy/region >/dev/null
+for link in /etc/environment.d/90-omarchy-china.conf /.cargo/config.toml /etc/gemrc; do
+  old=$(readlink "$link" || true)
+  if [[ $old == */default/region/china/* ]]; then
+    sudo ln -sfn "$OMARCHY_PATH/default/regions/cn/${old##*/}" "$link"
+  fi
+done