flush stdout logs (#2669)

else pipes end up not getting log output until much later

beacon_chain/nimbus_binary_common.nim

@@ -31,6 +31,7 @@ proc setupStdoutLogging*(logLevel: string) =
       proc (logLevel: LogLevel, msg: LogOutputStr) {.gcsafe, raises: [Defect].} =
         try:
           stdout.write(msg)
+          stdout.flushFile()
         except IOError as err:
           logLoggingFailure(cstring(msg), err)