Bump cirrusci/flutter from 3.3.3 to 3.3.4 in /.ci (#2695)

diff --git a/.ci/Dockerfile b/.ci/Dockerfile
index e0d4cbe..bebe096 100644
--- a/.ci/Dockerfile
+++ b/.ci/Dockerfile
@@ -2,7 +2,7 @@
 # before running. What matters is that the base image is pinned to minimize
 # unintended changes when modifying this file.
 # This is the hash for the 3.0.0 image.
-FROM cirrusci/flutter@sha256:f8a20768d3a04a214adfcdbeda70033863f0820f99c5dc5feaa6419bf54e65e3
+FROM cirrusci/flutter@sha256:794fbbce5422e79aae1dbf6e6cbfcf5fe89aff50350d7c018853b3cfdf35821a
 
 RUN apt-get update -y