Merge #5705

5705: Log the command flycheck runs to debug misconfigurations r=jonas-schievink a=Veetaha

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 (i.e. they have mutually-exclusive features).
Having the command rust-analyzer ran should help a lot

Co-authored-by: Veetaha <veetaha2@gmail.com>