Guard with RELEASE_BUILD_FLAGS_IN_PROTOBUF

If false, then we output an empty file, which will cause
release_config.mk to do the old process.

Bug: 328495189
Test: manual
Change-Id: I4aa82301f3bbb29633a275a801431a6667feb48d
diff --git a/cmd/release_config/crunch_flags/main.go b/cmd/release_config/crunch_flags/main.go
index 95342b1..4d763c8 100644
--- a/cmd/release_config/crunch_flags/main.go
+++ b/cmd/release_config/crunch_flags/main.go
@@ -21,6 +21,7 @@
 	manualFlagNamePrefixes []string = []string{
 		"RELEASE_ACONFIG_",
 		"RELEASE_PLATFORM_",
+		"RELEASE_BUILD_FLAGS_",
 	}
 
 	// Set `aconfig_flags_only: true` in these release configs.