It looks like we set `CARGO_HOME` incorrectly for invocations of both `cargo build` and `cargo test`. The `cargo build` invocations work because we first `cd` to the directory containing the `.cargo` directory. The `cargo test` invocations don't work for out-of-source-tree builds. legacy/trac#26455 contains a more minimal fix. The setting of `CARGO_HOME` is redundant if we `cd` to the correct directory first. Or we could stop changing directories and instead set `CARGO_HOME` correctly. We probably shouldn't do both because it's confusing and possibly inconsistent.