Re-add tag filter (flutter#5918)

Re-adding filter removed in flutter#5883 per [discussion](https://github.com/flutter/plugins/pull/5883/files#r887442326)

Author: stuartmorgan-g

.cirrus.yml

@@ -1,7 +1,7 @@
 gcp_credentials: ENCRYPTED[!ebad0a1f4f7a446b77944c33651460a7ab010b4617273cb016cf354eb8fc22aa92e37a3c58bfa4a0c40a799351e027a6!]
 
-# Run on PRs and main branch post submit only.
-only_if: $CIRRUS_PR != '' || $CIRRUS_BRANCH == 'main'
+# Run on PRs and main branch post submit only. Don't run tests when tagging.
+only_if: $CIRRUS_TAG == '' && ($CIRRUS_PR != '' || $CIRRUS_BRANCH == 'main')
 env:
   CHANNEL: "master" # Default to master when not explicitly set by a task.
   PLUGIN_TOOL: "./script/tool/bin/flutter_plugin_tools.dart"