Update brew before using it
diff --git a/tools/internal_ci/helper_scripts/prepare_build_macos_interop_rc b/tools/internal_ci/helper_scripts/prepare_build_macos_interop_rc
index bb046de..6ecf51d 100644
--- a/tools/internal_ci/helper_scripts/prepare_build_macos_interop_rc
+++ b/tools/internal_ci/helper_scripts/prepare_build_macos_interop_rc
@@ -25,6 +25,7 @@
 cd /Users/kbuilder/workspace/grpc
 
 # Needed for identifying Docker image sha1
+brew update
 brew install md5sha1sum
 
 # Set up gRPC-Go and gRPC-Java to test