FormatConverter: ensure we don't get stalled due to stdout buffer flood

this can happen due to very noisy outputs during conversion, e.g. if there were many unknown blocks.

src/world/format/io/FormatConverter.php

@@ -33,6 +33,7 @@ use function basename;
 use function crc32;
 use function file_exists;
 use function floor;
+use function flush;
 use function microtime;
 use function mkdir;
 use function random_bytes;
@@ -150,6 +151,7 @@ class FormatConverter{
 				$diff = $time - $thisRound;
 				$thisRound = $time;
 				$this->logger->info("Converted $counter / $count chunks (" . floor($this->chunksPerProgressUpdate / $diff) . " chunks/sec)");
+				flush();
 			}
 		}
 		$total = microtime(true) - $start;