'multirust update' doesn't work correctly after running `multirust env`

The overrides break things:

```
rustc 1.1.0-nightly (4b88e8f63 2015-05-11) (built 2015-05-12)
Process didn't exit successfully: `rustc -vV` (exit code: 1)
--- stderr
multirust: toolchain 'stable' not installed. run `multirust update stable` to install

```

Start by reproducing the failure with `multirust env`, then run `multirust update` and the reported `rustc -vV` command. Trace how the environment overrides interact with toolchain selection; done means updating works after `multirust env` and `rustc -vV` exits successfully.

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