[ci] fix ci failure on ios builds (#3470)

diff --git a/.cirrus.yml b/.cirrus.yml
index 4ec73ea..1f82270 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -191,6 +191,7 @@
         # https://github.com/flutter/flutter/issues/42864
         - if [[ "$CHANNEL" -eq "stable" ]]; then find . | grep _web$ | xargs rm -rf; fi
         - flutter channel $CHANNEL
+        - flutter upgrade
         - ./script/incremental_build.sh build-examples --ipa
         - ./script/incremental_build.sh drive-examples
         - ./script/incremental_build.sh xctest --target RunnerUITests --skip $PLUGINS_TO_SKIP_XCTESTS