LinuxMachineCpuUsageEmitter fails on macos

The `vmstat` command is not available on macos and it fails. Maybe use `top` on macos.

```c#
        else if (_job.OperatingSystem is OperatingSystem.Linux or OperatingSystem.OSX)
        {
            cpuEmitter = new LinuxMachineCpuUsageEmitter(Measurements.BenchmarksCpuGlobal, counterName: "vmstat");
        }
```