Fix the timestamp format when -logtimemicros is set

Summary:
The Z suffix is misplaced when the -logtimemicros option is set.
This case has been forgotten in D2425.

Test Plan:

make check

./src/bitcoind -logtimemicros -printtoconsole

Check the Z suffix is at the end of the timestamp.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3423