refactor: unify command declaration across codebase

plugins/checks/subcommands/default

@@ -6,11 +6,13 @@ source "$PLUGIN_AVAILABLE_PATH/checks/functions"
 
 cmd-checks-default() {
   declare desc="displays app zero-downtime checks status"
-  local cmd="checks"
+  declare cmd="checks"
   [[ "$1" == "$cmd" ]] && shift 1
+  declare APP="$1"
+
   local APPS=$(dokku_apps)
-  if [[ -n "$1" ]]; then
-    local APPS="$1"
+  if [[ -n "$APP" ]]; then
+    local APPS="$APP"
   fi
 
   dokku_log_warn "Deprecated: Please use checks:report"