floss: Fix build.rs for bt facade protos

Proto files were moved into the Blueberry directory but the linux build
wasn't updated to match.

This fixes e9bb55a0 (Fix local GD cert runner).

Bug: 218339046
Tag: #floss
Test: emerge-zork floss and ./build.py

Change-Id: I174835367679fba26a18d16c71313b751a4fa910