fix(installer): resolve multi-disk LUKS/BTRFS boot hang

- Move temporary LUKS keyfile to /tmp/ so Disko omits it from runtime config
- Explicitly add x-systemd.requires and x-systemd.device-timeout=0 to BTRFS mount options
- Ensures all LUKS devices are decrypted before BTRFS attempts to mount

installer/install.sh

@@ -1089,7 +1089,7 @@ prewipe_target_drive() {
     success "Pre-wipe complete"
 }
 
-_LUKS_KEY_PATH="/dev/shm/nomarchy-luks.key"
+_LUKS_KEY_PATH="/tmp/nomarchy-luks.key"
 
 # Wrap the disko invocation so a failure surfaces the last few lines of
 # output and offers Retry / View full log / Abort. set -e is suspended for