fix(resolvers): put DNSStubListener=no in resolved.conf.d, not unit dropins

Butane unit dropins write to /etc/systemd/system/<unit>.d/, but
DNSStubListener is a systemd-resolved config option that belongs in
/etc/systemd/resolved.conf.d/. Use storage.files instead so Ignition
places it in the correct location.

tofu/flatcar-resolver.bu

@@ -25,6 +25,12 @@ storage:
         inline: |
           nameserver 1.1.1.1
           nameserver 9.9.9.9
+    - path: /etc/systemd/resolved.conf.d/no-stub.conf
+      mode: 0644
+      contents:
+        inline: |
+          [Resolve]
+          DNSStubListener=no
     - path: /etc/systemd/network/00-eth0.network
       mode: 0644
       contents:
@@ -118,12 +124,6 @@ storage:
 
 systemd:
   units:
-    - name: systemd-resolved.service
-      dropins:
-        - name: no-stub.conf
-          contents: |
-            [Resolve]
-            DNSStubListener=no
     - name: docker.service
       enabled: true
     - name: unbound.service

tofu/flatcar-resolver2.bu

@@ -25,6 +25,12 @@ storage:
         inline: |
           nameserver 1.1.1.1
           nameserver 9.9.9.9
+    - path: /etc/systemd/resolved.conf.d/no-stub.conf
+      mode: 0644
+      contents:
+        inline: |
+          [Resolve]
+          DNSStubListener=no
     - path: /etc/systemd/network/00-eth0.network
       mode: 0644
       contents:
@@ -118,12 +124,6 @@ storage:
 
 systemd:
   units:
-    - name: systemd-resolved.service
-      dropins:
-        - name: no-stub.conf
-          contents: |
-            [Resolve]
-            DNSStubListener=no
     - name: docker.service
       enabled: true
     - name: unbound.service