worker: do not look up context twice in PostMessage

Refs: https://github.com/nodejs/node/issues/38780#issuecomment-846548949

PR-URL: https://github.com/nodejs/node/pull/38784


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>