Switch master to main in prepare_tool.sh (#2200)

diff --git a/.ci/scripts/prepare_tool.sh b/.ci/scripts/prepare_tool.sh
index b25d769..2e17392 100755
--- a/.ci/scripts/prepare_tool.sh
+++ b/.ci/scripts/prepare_tool.sh
@@ -4,7 +4,7 @@
 # found in the LICENSE file.
 
 # To set FETCH_HEAD for "git merge-base" to work
-git fetch origin master
+git fetch origin main
 
 # Pinned version of the plugin tools, to avoid breakage in this repository
 # when pushing updates from flutter/plugins.