fix: escape shell expansion in kubeadm helper scripts

nixos/kubeadm/flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1767313136,
+        "narHash": "sha256-16KkgfdYqjaeRGBaYsNrhPRRENs0qzkQVUooNHtoy2w=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "ac62194c3917d5f474c1a844b6fd6da2db95077d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-25.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

nixos/kubeadm/modules/k8s-common.nix

@@ -92,7 +92,7 @@ in
       domain="${config.terrahome.kubeadm.clusterDomain}"
 
       local_ip_cidr=$(ip -4 -o addr show dev "$iface" | awk 'NR==1 {print $4}')
-      if [ -z "${local_ip_cidr:-}" ]; then
+      if [ -z "''${local_ip_cidr:-}" ]; then
         echo "Could not determine IPv4 CIDR on interface $iface"
         exit 1
       fi
@@ -142,7 +142,7 @@ in
       iface="${config.terrahome.kubeadm.controlPlaneInterface}"
       suffix="${toString config.terrahome.kubeadm.controlPlaneVipSuffix}"
       local_ip_cidr=$(ip -4 -o addr show dev "$iface" | awk 'NR==1 {print $4}')
-      if [ -z "${local_ip_cidr:-}" ]; then
+      if [ -z "''${local_ip_cidr:-}" ]; then
         echo "Could not determine IPv4 CIDR on interface $iface"
         exit 1
       fi