`test_rlib_dep_reader_call` asserts `env::var("CARGO_HOME").is_ok()` but it's not necessarily true

.. not for Debian shipped cargo, apparently: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1090040

Remotely related to https://github.com/mozilla/sccache/pull/2298.

A patch using an unqualified `Path::from("rustc")` works ™, though is probably not _good_.