trust anchors: use parallel-safe temporary name

Previously multiple kresd processes might use the same .lock file at once and thus have a race between writing and renaming. That could happen relatively often if starting many instances at once.

mentioned in issue #389 (closed)

TODO:

• NEWS entry (will probably conflict with knot-2.7)

enabled an automatic merge when the pipeline for 9776f3fb succeeds

merged

mentioned in commit ff9a39f9