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

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