MINOR: extcheck: pass the master and worker IDs to external checks

MINOR: extcheck: pass the master and worker IDs to external checks

By default, i.e. unless "external-check preserve-env" is used, the
environment of an external check is not inherited but entirely rebuilt
from the curated list in extcheck_envs[], so the two new identifiers
would not be visible from a check script. Let's add them there, next to
PATH, as a new process-scoped group before the proxy and server ones.
It's also cleaner because we pass the real directives, not the env
variables which could have been modified.