tor-circmgr: Exclude the target relay when building vanguards circuits.

Some of the circuit post-build checks added in !2181 (merged) and !2183 (merged) were failing in shadow, but they went unnoticed because they weren't causing the simulation to fail. In ec182456, I extended the shadow CI script with a temporary fix, which makes the simulation fail if any of the arti processes logs an internal error.

f456d8c5 and 736fa9a3 update the vanguard circuit building code to ensure neither of the last 2 hops are the same as the circuit target. This fixes the failing post-build checks from ec182456.

This should be the last Blocker for the 1.2.4 release.