fix(resolvers): disable systemd-resolved stub listener instead of masking

systemd-resolved must run for host DNS (nss/resolv.conf), but its stub
listener on 127.0.0.53:53 conflicts with Unbound's bind to 0.0.0.0:53.

Replace masking with DNSStubListener=no dropin + static resolv.conf
pointing to 1.1.1.1 and 9.9.9.9, so the host can resolve without
occupying port 53.

tofu/flatcar-resolver.bu

@@ -18,6 +18,13 @@ storage:
       overwrite: true
       contents:
         inline: flatcar-resolver1
+    - path: /etc/resolv.conf
+      mode: 0644
+      overwrite: true
+      contents:
+        inline: |
+          nameserver 1.1.1.1
+          nameserver 9.9.9.9
     - path: /etc/systemd/network/00-eth0.network
       mode: 0644
       contents:
@@ -30,8 +37,8 @@ storage:
           [Network]
           Address=204.110.191.240/26
           Gateway=204.110.191.254
-          DNS=9.9.9.9
           DNS=1.1.1.1
+          DNS=9.9.9.9
 
           [Address]
           Address=2606:1c80::240/64
@@ -112,7 +119,11 @@ storage:
 systemd:
   units:
     - name: systemd-resolved.service
-      mask: true
+      dropins:
+        - name: no-stub.conf
+          contents: |
+            [Resolve]
+            DNSStubListener=no
     - name: docker.service
       enabled: true
     - name: unbound.service

tofu/flatcar-resolver2.bu

@@ -18,6 +18,13 @@ storage:
       overwrite: true
       contents:
         inline: flatcar-resolver2
+    - path: /etc/resolv.conf
+      mode: 0644
+      overwrite: true
+      contents:
+        inline: |
+          nameserver 1.1.1.1
+          nameserver 9.9.9.9
     - path: /etc/systemd/network/00-eth0.network
       mode: 0644
       contents:
@@ -30,8 +37,8 @@ storage:
           [Network]
           Address=204.110.191.250/26
           Gateway=204.110.191.254
-          DNS=9.9.9.9
           DNS=1.1.1.1
+          DNS=9.9.9.9
 
           [Address]
           Address=2606:1c80::250/64
@@ -112,7 +119,11 @@ storage:
 systemd:
   units:
     - name: systemd-resolved.service
-      mask: true
+      dropins:
+        - name: no-stub.conf
+          contents: |
+            [Resolve]
+            DNSStubListener=no
     - name: docker.service
       enabled: true
     - name: unbound.service