systemd-boot-builder: add missing fsync in atomic writes

write(2) and close(2) doesn't ensure the file content actually got synched, so let's also fsync before doing the rename

nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

···
         if not @editor@:
             f.write("editor 0\n")
         f.write("console-mode @consoleMode@\n")
+        f.flush()
+        os.fsync(f.fileno())
     os.rename("@efiSysMountPoint@/loader/loader.conf.tmp", "@efiSysMountPoint@/loader/loader.conf")
 
 
···
                     description=describe_generation(profile, generation, specialisation)))
         if machine_id is not None:
             f.write("machine-id %s\n" % machine_id)
+        f.flush()
+        os.fsync(f.fileno())
     os.rename(tmp_path, entry_file)