`radicle-cli-test` initiates `cargo` builds under the assumption
that the target directory is always within the source directory.
However, the user may have a different idea, specified with the
environment variable `CARGO_TARGET_DIR` (supported by `cargo`, see
https://doc.rust-lang.org/cargo/reference/environment-variables.html).
Add support for `CARGO_TARGET_DIR`, with a fallback to 'target'
at the top of the source tree if `CARGO_TARGET_DIR` is undefined.
adfd89f609