Use independent `target_dir` for the `test`

[qryxip]

Hello from rust-analyzer! We might still revert the change that prompted this, but this PR seems like a good idea anyway.

My suggestion here would be to set a different target directory (CARGO_TARGET_DIR) for cargo-compete. If you use the same one, rust-analyzer's check on save and cargo-compete will block each other and overwrite their build artifacts.

Originally posted by @lnicola in #172 (comment)

[lnicola]

Since maybe it's not obvious, rust-analyzer itself can already be configured to use a different target directory (e.g. by using server.extraEnv), but since #172 cargo-compete uses a different toolchain from what cargo would use, so it makes sense to use its own target directory too.