Fix codesign build validation.

This build was failing because "git checkout" requires a "git fetch"
first.

Bug: https://github.com/flutter/flutter/issues/96889
Change-Id: I9f24b66248f1cc1781f7cabcbc69d07cce6de80a
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/25501
Reviewed-by: Christopher Fujino <fujino@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
diff --git a/recipe_modules/adhoc_validation/resources/verify_binaries_codesigned.sh b/recipe_modules/adhoc_validation/resources/verify_binaries_codesigned.sh
index 150be1c..6e4ea13 100644
--- a/recipe_modules/adhoc_validation/resources/verify_binaries_codesigned.sh
+++ b/recipe_modules/adhoc_validation/resources/verify_binaries_codesigned.sh
@@ -4,4 +4,10 @@
 
 set -e
 
+# Checking out master here is ok even though this test runs on release branches
+# because the checkout is used only to download the conductor tool but the validation
+# is run on the binaries for the commit passed as property to the builder.
+git fetch origin master:master
+
+# Run the actual validation.
 ./dev/conductor/bin/conductor codesign --verify