time: Make SystemTime::now() use the correct time unit

The previous conversion stored the timestamp as nanoseconds, instead of
microseconds as the codebase expects.

According to MDN:

> The Date.now() static method returns the number of milliseconds
elapsed since the epoch

<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now>

The issue has been detected while comparing SystemTime::now() timestamps
generated by a Linux executable with timestamps generated by kywasmtime.

SystemTime::now().as_micros() results:

 * Linux std::time::SystemTime:  1738590492673999872
 * kywasmtime::time::SystemTime: 1738590493317670

sleep: Remove commented line