Log the command flycheck runs to debug misconfigurations

Log the command flycheck runs to debug misconfigurations

Without this users have no clue why flycheck fails to run.
This is what is printed to the output channel:
```
[ERROR rust_analyzer::main_loop] cargo check failed: Cargo watcher failed,the command produced no valid metadata (exit code: ExitStatus(ExitStatus(25856)))
```

I stumbled with this figuring out that rust-analyzer adds `--all-features` which is not intended
for some crates in the workspace (e.g. they have mutually-exclusive features.
Having the command rust-analyzer ran should help a lot