Only install snapshotctl on debug builds

snapshotctl is a debug tool, which is intended for engineers to inspect
snapshot partition state when OTA fails. It is not meant for production
use, since it requires root, it can't be used in prod devices' adb shell
either.

Test: build, make sure build has snapshotctl
Change-Id: I350b9522f6c18606c3f3c15a8c231a3c41bee18f