bootctl: do not use the EFI path as XBOOTLDR path

bootctl: do not use the EFI path as XBOOTLDR path

When running in a container, EFI and XBOOTLDR partition check is
relaxed, hence /boot may be recognized as both EFI and XBOOTLDR
partition.

Before:
```
$ run0 systemd-nspawn -xD / --private-network --bind=/sys/firmware/efi/efivars --bind=/boot -E SYSTEMD_LOG_LEVEL=debug -- build/bootctl --variables=yes --no-pager -x
Failed to check file system type of "/efi": No such file or directory
Using EFI System Partition at /boot.
Using XBOOTLDR partition at /boot as $BOOT.
/boot
```

After:
```
$ run0 systemd-nspawn -xD / --private-network --bind=/sys/firmware/efi/efivars --bind=/boot -E SYSTEMD_LOG_LEVEL=debug -- build/bootctl --variables=yes -x
Failed to check file system type of "/efi": No such file or directory
Using EFI System Partition at /boot.
Didn't find an XBOOTLDR partition, using the ESP as $BOOT.
/boot
```