refactor: switch detected builder so first one wins

Also rename internal cnb references to pack (where possible).

plugins/builder-pack/builder-detect

@@ -9,12 +9,12 @@ trigger-builder-pack-builder-detect() {
   declare APP="$1" SOURCECODE_WORK_DIR="$2"
 
   if [[ -f "$SOURCECODE_WORK_DIR/project.toml" ]]; then
-    echo "cnb"
+    echo "pack"
     return
   fi
 
   if [[ "$(config_get "$APP" DOKKU_CNB_EXPERIMENTAL || true)" == "1" ]]; then
-    echo "cnb"
+    echo "pack"
     return
   fi
 }