build: bump Go toolchain to 1.26.5

Tailscale HEAD go.mod now requires go >= 1.26.5; build images pinned
1.26.4. Bump go.mod, the four Go Dockerfiles, and the nixpkgs pin
(staging-next-26.05 ships go_1_26 1.26.5; unstable still lags).

(cherry picked from commit 8eec2dbdc2)

flake.lock

@@ -1,5 +1,27 @@
 {
   "nodes": {
+    "flake-checks": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "treefmt-nix": "treefmt-nix"
+      },
+      "locked": {
+        "lastModified": 1783015821,
+        "narHash": "sha256-vmzYTZxIAy3OYXwkRL5GQYk8fNnh8TLWcxQRIO3ywOU=",
+        "owner": "kradalby",
+        "repo": "flake-checks",
+        "rev": "3c821706eb0bd07f515f9a7726b650ccae927433",
+        "type": "github"
+      },
+      "original": {
+        "owner": "kradalby",
+        "repo": "flake-checks",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -20,11 +42,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1781153106,
-        "narHash": "sha256-yzsroLCcuRG4KdGMxWt0eXKOrRSgQT8/xjYngeq9ujU=",
+        "lastModified": 1784421286,
+        "narHash": "sha256-YKn3t7FvaargtyNHbrOw+qzDM+93q5kSCREUPNxeZQs=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "9ee75f111a06d7ab2b2f729698a8eff53d54e070",
+        "rev": "62eaae30cd05c04718500ddb44e7ef85cd45ddd5",
         "type": "github"
       },
       "original": {