gh: test for existence of hosts file

Having the module enabled but never using gh will result in the config file
existing but no hosts.yml. In that scenario we won't have anything to
migrate, so only test for hosts.yml.

modules/programs/gh.nix

@@ -136,7 +136,7 @@ in {
     # See https://github.com/nix-community/home-manager/issues/4744 for details.
     home.activation.migrateGhAccounts =
       hm.dag.entryBetween [ "linkGeneration" ] [ "writeBoundary" ] ''
-        if [[ -e "${config.xdg.configHome}/gh/config.yml" ]]; then
+        if [[ -e "${config.xdg.configHome}/gh/hosts.yml" ]]; then
           (
             TMP_DIR=$(mktemp -d)
             trap "rm --force --recursive $TMP_DIR" EXIT