Describe the problem you are trying to solve
A number of 3rd party subcommands exist that provide extended behaviours such as rinstall or cargo-native-install which can install content built from the target-dir into the host system in various places or extended ways. Today, these have to infer target-dir either from a CLI switch, or attempt to recreate cargo's interpretation of CARGO_TARGET_DIR and the various .cargo/config files that may exist.

Describe the solution you'd like
When running 3rd party subcommands, cargo should expose an environment variable to these which contains the processed target-dir allowing inheritance of cargos processing of this variable.