process: fix omitting `--` from `process.execArgv`

This was essentially a typo that went unnoticed because we
didn’t have tests for this particular situation.

Fixes: https://github.com/nodejs/node/issues/24647

PR-URL: https://github.com/nodejs/node/pull/24654


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>