stages/rpm: make the fake machine-id newline-terminated

From machine-id(5): machine ID is a single *newline-terminated*, hexadecimal,
32-character, lowercase ID.

This commit fixes that by adding a newline.

This broke us on Fedora 38:
This pull request added `set -e` to kernel-install:

https://github.com/systemd/systemd/pull/23881

This caused F38 pipeline failing because the script also runs
`read -r MACHINE_ID </etc/machine-id`. `read` returns 1 if the input doesn't
contain a newline, thus after the addition of `set -e`, the script started
crashing with our bad machine-id. This caused crashing of the whole image build
in the grub2 stage which requires kernel-install to work (because kernel-install
is responsible for putting vmlinuz into /boot).

stages/org.osbuild.rpm

@@ -272,7 +272,7 @@ def create_machine_id_if_needed(tree):
     os.makedirs(f"{tree}/etc", exist_ok=True)
     with open(path, "w", encoding="utf8") as f:
         # create a fake machine ID to improve reproducibility
-        f.write("ffffffffffffffffffffffffffffffff")
+        f.write("ffffffffffffffffffffffffffffffff\n")
         os.fchmod(f.fileno(), 0o400)
 
     return True