report: use consistent format for dumpEventTime

Use the same JS-compatible format on both POSIX and Windows.

PR-URL: https://github.com/nodejs/node/pull/25751


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>