bootctl: prettify tree output of discovered boot loaders a bit

bootctl: prettify tree output of discovered boot loaders a bit

Previously, the tree output of discovered boot laoders in the ESP would
look like this:

Available Boot Loaders on ESP:
          ESP: /efi (/dev/disk/by-partuuid/0c6f41ed-2573-4723-8c84-23681f9d1c28)
         File: └─/EFI/systemd/systemd-bootx64.efi (systemd-boot v250.7-1.fc36)
         File: └─/EFI/BOOT/BOOTX64.EFI (systemd-boot v250.7-1.fc36)

With this change the tree branches are corrected to look like this:

Available Boot Loaders on ESP:
          ESP: /efi (/dev/disk/by-partuuid/0c6f41ed-2573-4723-8c84-23681f9d1c28)
         File: ├─/EFI/systemd/systemd-bootx64.efi (systemd-boot v250.7-1.fc36)
               └─/EFI/BOOT/BOOTX64.EFI (systemd-boot v250.7-1.fc36)

Ahhh! So much nicer. This incorrect tree drawing has been bugging me for
so long. Finally I can sleep at night again!