do not run osm2pgsql append with mutliple threads

As the updates modify the placex table, there may be deadlocks
when different objects want to forward modifications to the same
place (for example because they are both linked to it).

nominatim/tools/exec_utils.py

@@ -119,7 +119,7 @@ def run_osm2pgsql(options: Mapping[str, Any]) -> None:
     cmd = [str(options['osm2pgsql']),
            '--hstore', '--latlon', '--slim',
            '--log-progress', 'true',
-           '--number-processes', str(options['threads']),
+           '--number-processes', '1' if options['append'] else str(options['threads']),
            '--cache', str(options['osm2pgsql_cache']),
            '--style', str(options['osm2pgsql_style'])
           ]