branding: also set the UKI format

This is a workaround for systemd/mkosi#3299

Author: davide125

mkosi.profiles/branding/mkosi.conf

@@ -9,6 +9,9 @@ Packages=
         plymouth-theme-hot-dog
 InitrdPackages=
         plymouth-theme-hot-dog
+# This is hardcoded as otherwise it'll use ParticleOS regardless of what
+# ImageId we set here: https://github.com/systemd/mkosi/issues/3299
+UnifiedKernelImageFormat=ArcadeOS-%v
 
 [Output]
 ImageId=ArcadeOS