fix: cannot debug child forked process

[ulivz]

Summary

Since all umi instance are spawned via child_process.fork, but execArgv will be default to process.execArgv, causing the parent and child process used the same inspecting port, which takes following error:

Starting inspector on 127.0.0.1:9229 failed: address already in use

This PR doesn't introduced the mechanism of checking the usage of port like node-portfinder, it just ensures:

1. Parent and child process won't share the same inspecting port;
2. Multiple child processes won't share the same inspecting port.

Refs:

• Debugger doesn't work for processes which fork other processes nodejs/node#9435
• inspector: SIGUSR1 should use next available port if 9922 is already used nodejs/node#16872
• child_process.fork(modulePath[, args][, options])

[ulivz]

@sorrycc

Could you help me to take a look, it blocked me to debug when I re-install all packages and I need to change the code manually each time. Thanks.

[sorrycc]

Thanks for PR!