tests: ensure that all builds are made in the same target directory — heartwood

‘build test’ builds radicle-remote-helper and radicle-cli, making 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).

This change adds support for CARGO_TARGET_DIR, with a fallback to ‘target’ at the top of the source tree if CARGO_TARGET_DIR is undefined.

             for (package, binary) in binaries {
                 log::debug!(target: "test", "Building binaries for package `{package}`..");
                 let cargo_manifest_dir = cargo_manifest_dir();
                 let results = escargot::CargoBuild::new()
                     .package(package)
                     .bin(binary)
                     .manifest_path(cargo_manifest_dir.clone().join("Cargo.toml"))
                     .target_dir(cargo_manifest_dir.join(CARGO_TARGET_DIR_DIRNAME))
                     .manifest_path(cargo_manifest_dir().join("Cargo.toml"))
                     .target_dir(cargo_target_dir())
                     .exec()
                     .unwrap();

     env::var("CARGO_MANIFEST_DIR").map(PathBuf::from).unwrap()
 }
 fn cargo_target_dir() -> PathBuf {
     env::var("CARGO_TARGET_DIR")
         .map(PathBuf::from)
         .unwrap_or(cargo_manifest_dir().join(CARGO_TARGET_DIR_DIRNAME))
 }
 /// Get the list of binary paths to use as `$PATH` for the tests,
 /// starting with the current working directory.
 fn bins(cwd: PathBuf) -> Vec<PathBuf> {

     // this makes it more convenient to execute scripts during testing.
     bins.push(cwd);
     bins.push(
         cargo_manifest_dir()
             .join(CARGO_TARGET_DIR_DIRNAME)
             .join(CARGO_PROFILE),
     );
     bins.push(cargo_target_dir().join(CARGO_PROFILE));
     // Add the "real" `$PATH`.
     if let Ok(path) = env::var("PATH") {