Floss: Fix rerun-if-changed for topshim

rerun-if-changed should not contain `-I`, and only the topmost directory
is needed because cargo scans for the directory recursively.

Bug: 221310414
Tag: #floss
Test: Manual - `cargo build` and check that topshim is only built once
unless a file inside /system is touched.

Change-Id: I4558d260498c79bc0a6d5362e36504159464c918