Can I set the compiler used with a feature flag? I have a version of a library that has different behaviors depending on whether or not the MCU has an FPU. To support this I added the following to the .cargo/config.toml file:
```toml
[build.'cfg(not(feature = "lux_as_f32"))']
target = "thumbv7em-none-eabi"

[build.'cfg(feature = "lux_as_f32")']
target = "thumbv7em-none-eabihf"
```
But I am concerned this is not working, as I am seeing the following warning:
```
warning: unused config key `build.cfg(feature = "lux_as_f32")` in `<elided>/.cargo/config.toml`
```
(The library is working, but I am not sure if this is only because conditionally compiled code in the library is avoiding FPU calls in my usage on a non-FPU MCU.)