[backport#16995] refactor: Work around GCC 9 -Wredundant-move warning

Summary:
Use std::move workaround for unique_ptr, for when the C++ compiler lacks
a fix for this issue:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579
Do this in a way that avoids a GCC 9 -Wredundant-move warning.

https://github.com/bitcoin/bitcoin/pull/16995/commits/ff9c671b11d40e5d0623eff3dd12e48cbaafb34e

This completes the backport of Core PR16995

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6165