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
```

src/bootctl/bootctl-install.c

@@ -880,7 +880,7 @@ static int install_variables(
         if (r == -ENOENT)
                 return 0;
         if (r < 0)
-                return log_error_errno(r, "Cannot access \"%s/%s\": %m", esp_path, path);
+                return log_error_errno(r, "Cannot access \"%s/%s\": %m", esp_path, skip_leading_slash(path));
 
         r = find_slot(uuid, path, &slot);
         if (r < 0) {