Honor the rustfmt setting in config.toml

Honor the rustfmt setting in config.toml

Prior to this, setting the rustfmt configuration was ignored:

```
% mkdir example

% cd example

% ../configure --set build.rustfmt=/usr/bin/true

% ../x.py fmt
./x.py fmt is not supported on this channel
failed to run: /Users/shep/Projects/rust/example/build/bootstrap/debug/bootstrap fmt
Build completed unsuccessfully in 0:00:01
```

And after:

```
% ../x.py fmt
Build completed successfully in 0:00:11
```