floss: Add rerun-if-changed cargo instruction when using bt_packetgen

This makes cargo rerun the build script if the source file change

Fix: 249187187
Tag: floss
Test: ./build.py --target test
Test: manual
Bug: 245578454
Ignore-AOSP-First: cherry-pick from AOSP
Merged-In: I3e696dd3f3e98fa7bccbd0a1bdd02ac2e04f9fd4
Change-Id: I3e696dd3f3e98fa7bccbd0a1bdd02ac2e04f9fd4
(cherry picked from commit 1e291432)