rust: Pin protobuf to crate version 2.27.1 am: 6b6e0230eb

Original change: https://android-review.googlesource.com/c/trusty/app/keymint/+/2753350

Change-Id: I6d3fd4cd7fe84f110a3040dda67c36e11937acee
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>