Replace incorrect template to update

The incorrect variable was telling the system to update
fedora-40 (which does not exist in default systems) instead
of fedora-40-xfce.

Fixes #1166

securedrop_salt/sd-sys-vms.sls

@@ -32,7 +32,7 @@ set-fedora-template-as-default-mgmt-dvm:
 # If the VM has just been installed via package manager, update it immediately
 update-fedora-template-if-new:
  cmd.wait:
- - name: qubes-vm-update --just-print-progress --show-output --targets {{ sd_supported_fedora_version }}
+ - name: qubes-vm-update --just-print-progress --show-output --targets {{ sd_fedora_base_template }}
  - require:
  - cmd: dom0-install-fedora-template
  # Update the mgmt-dvm setting first, to avoid problems during first update