cluster: fix inspector port assignment

Make sure that inspector ports in cluster are inside the valid range:
`[1024, 65535]`.
Fixes flaky `test-inspector-port-zero-cluster`.

PR-URL: https://github.com/nodejs/node/pull/18696
Fixes: https://github.com/nodejs/node/issues/18303


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>