stages/dnf: Improve dnf stage reproducibility

Normally, machine ID is generated randomly when dnf installs @Core
group. Unfortunately this isn't helping us with reproducibility
of images.

This commit introduces the concept of fake machine ID. Before dnf
command is run in dnf stage, we set the machine ID to a fake one.
This ensures all the scriptlets requiring machine ID have predictable
outputs.

For example GRUB uses machine-id in file names inside
/boot/loader/entries. With fixed machine ID the file names are always
the same and totally predictable.

stages/org.osbuild.dnf

@@ -3,6 +3,7 @@
 import hashlib
 import json
 import os
+import pathlib
 import subprocess
 import sys
 
@@ -71,6 +72,17 @@ def main(tree, options):
         mount -o bind /sys {tree}/sys
         mount -o bind /proc {tree}/proc
     """
+
+    machine_id_set_previously = os.path.exists(f"{tree}/etc/machine-id")
+    if not machine_id_set_previously:
+        # create a fake machine ID to improve reproducibility
+        print("creating a fake machine id")
+        script += f"""
+            mkdir -p {tree}/etc
+            echo "ffffffffffffffffffffffffffffffff" > {tree}/etc/machine-id
+            chmod 0444 {tree}/etc/machine-id
+        """
+
     try:
         subprocess.run(["/bin/sh", "-c", script], check=True)
     except subprocess.CalledProcessError as err:
@@ -110,6 +122,13 @@ def main(tree, options):
             os.rmdir(name, dir_fd=dirfd)
     os.close(fd)
 
+    # remove temporary machine ID if it was created by us
+    if not machine_id_set_previously:
+        print("deleting the fake machine id")
+        machine_id_file = pathlib.Path(f"{tree}/etc/machine-id")
+        machine_id_file.unlink()
+        machine_id_file.touch()
+
     return 0