CreateNewBlock: insert entries into block slightly earlier so that correct size is logged

Summary:
I noticed the log message was always printing "CreateNewBlock(): total size: 81 ...",
which is a bit silly.

This moves the pblock population back to where it belongs, right after the
entries are determined. To make this work, the coinbase tx is inserted
into pblock directly which is also what we want (and this goes back to what
Core does).

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5019