LShift makes LAlt's KeyCode None on Linux

## Bevy version
b964024

## Operating system & version
Pop!_OS 20.10 (Based on Bunutu 20.10)

## What you did
Ran this code:
```rust
use bevy::{input::keyboard::KeyboardInput, prelude::*};

fn main() {
App::build()
.insert_resource(WindowDescriptor::default())
.add_plugins(DefaultPlugins)
.add_system(update.system())
.run();
}

fn update(mut keyboard_input_events: EventReader) {
for event in keyboard_input_events.iter() {
println!("{:?}", event);
}
}
```

And then press Left Alt while holding Left Shift.

## What you expected to happen
The console to print the following:
```text
KeyboardInput { scan_code: 42, key_code: Some(LShift), state: Pressed }
KeyboardInput { scan_code: 56, key_code: Some(LAlt), state: Pressed }
```

## What actually happened
The console prints the following:
```text
KeyboardInput { scan_code: 42, key_code: Some(LShift), state: Pressed }
KeyboardInput { scan_code: 56, key_code: None, state: Pressed }
```

## Additional information
I am unable to reproduce this bug on Windows.