nixos/systemd-boot: strip newline from machine-id

To make each line in the entry file consistent, strip the newline from
the machine-id content.

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

@@ -277,7 +277,7 @@ def get_profiles() -> list[str]:
 def install_bootloader(args: argparse.Namespace) -> None:
     try:
         with open("/etc/machine-id") as machine_file:
-            machine_id = machine_file.readlines()[0]
+            machine_id = machine_file.readlines()[0].strip()
     except IOError as e:
         if e.errno != errno.ENOENT:
             raise