bootctl: do not print slash more than once

bootctl: do not print slash more than once

When bootctl is called by an unprivileged user, then previously we got
```
Failed to read "/boot/EFI/systemd": Permission denied
Failed to open '/boot//loader/loader.conf': Permission denied
```
Now, with this patch, we get
```
Failed to read "/boot/EFI/systemd": Permission denied
Failed to open '/boot/loader/loader.conf': Permission denied
```