Avoid weaker comparison for targets in `run_cargo`

[adpaco]

Proposed change: We should revisit the same_target check in run_cargo after a while to see if it's not needed anymore or it can be restricted to certain cases. This newer check was introduced in #3102 during a toolchain upgrade.

Motivation: The previous comparison was rustc_artifact == *target, but it started to fail in some cases after the cargo change in rust-lang/cargo#12783.