[skip ci] don't run Cirrus on tags (flutter#2396)

When all plugins are release at once there will be a tag for each of the plugins but each tag will again test all plugins which makes it not optimal.

.cirrus.yml

@@ -1,4 +1,6 @@
 task:
+  # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins
+  only_if: $CIRRUS_TAG == ''
   use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true' && $CIRRUS_PR == ''
   container:
     dockerfile: .ci/Dockerfile
@@ -91,6 +93,8 @@ task:
         - export CIRRUS_COMMIT_MESSAGE=`cat /tmp/cirrus_commit_message.txt`
 
 task:
+  # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins
+  only_if: $CIRRUS_TAG == ''
   use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true'
   osx_instance:
     image: mojave-xcode-11.2.1-flutter
@@ -140,6 +144,8 @@ task:
         - ./script/incremental_build.sh build-examples --ipa
         - ./script/incremental_build.sh drive-examples
 task:
+  # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins
+  only_if: $CIRRUS_TAG == ''
   use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true'
   osx_instance:
     image: mojave-xcode-11.2.1-flutter