build: change CANTRIP_OUT_DIR pathname construction

Change the output directory pathname to avoid collision between platforms;
specifically the "nexus" and "shodan" platforms. Build artifacts are now
written to:

$OUT/cantrip/<platform>/{debug,release}

with follow-on changes moving build artifacts into this area to (more
correctly) isolate builds.

Bug: 293296515

Change-Id: Ia1f38325c57a656e53d4973734b844fa296e46ec