Pin linux-zen kernel in archinstall config, add quiet boot param

- Set kernels to linux-zen in user_configuration.json to skip
  interactive kernel selection during install
- Add post-install step to append quiet to non-fallback systemd-boot
  entries for clean boot output

config/user_configuration.json

@@ -8,6 +8,8 @@
 
     "bootloader": "Systemd-boot",
 
+    "kernels": ["linux-zen"],
+
     "hostname": "moonarch",
 
     "locale_config": {