Multirust doesn't work with Cargo's tests

After installing multirust, I executed `make test` in cargo's repository.
But nearly all tests fail because multirust prints:

```
multirust: no default toolchain configured. run `multirust help default`
multirustproxy: `multirust check-updates-override` failed

```

Even after runnning `multirust default nightly`, I still get these errors.

Note that running `rustc` anywhere on my filesystem works (including `/path/to/cargo/target/x86_64-unknown-linux-gnu/cit/...`, which is where cargo puts his temporary files during the tests), so I have no idea where this problem comes from.