Floss: Check for C++ output dir instead of Rust output dir

To determine whether to re-run build script for btadapterd, check the
C++ output dir instead of Rust output dir, otherwise we will always
rebuild btadapterd unconditionally.

Bug: 203696027
Tag: #floss
Test: Manual - cargo build and check that btadapterd is only built once
unless there is a change in C++ output dir

Change-Id: I54fe4293a64d0d1abad3828d12590b63650d4ee6