restic: pin to 0.18.0

Let's keep the version of `restic` used by the prune job in sync with
the latest version in Fedora.

restic/kustomization.yaml

@@ -36,6 +36,7 @@ patches:
             spec:
               containers:
               - name: restic-prune
+                imagePullPolicy: IfNotPresent
                 env:
                 - name: RESTIC_CACERT
                   value: /run/dch-ca/dch-root-ca.crt
@@ -48,3 +49,6 @@ patches:
                 configMap:
                   name: dch-root-ca
 
+images:
+- name: ghcr.io/restic/restic
+  newTag: 0.18.0