rust: Pin protobuf to crate version 2.27.1

Bug: 281857510
Change-Id: I64777110cb265c6bb9266c8f53219290c8227216
diff --git a/rules.mk b/rules.mk
index 4f9ac18..47e34e5 100644
--- a/rules.mk
+++ b/rules.mk
@@ -33,7 +33,7 @@
 	trusty/user/base/lib/keymint-rust/common \
 	trusty/user/base/lib/keymint-rust/ta \
 	$(call FIND_CRATE,log) \
-	$(call FIND_CRATE,protobuf) \
+	$(call FIND_CRATE,protobuf)/2.27.1 \
 	trusty/user/base/lib/storage/rust \
 	trusty/user/base/lib/tipc/rust \
 	trusty/user/base/lib/system_state/rust \