flake: bump nixpkgs (sunshine 2025.924 -> 2026.516, security fix)

nixpkgs 2026-06-10 -> 2026-06-29. Pulls in Sunshine 2026.516.143833,
which ships critical security fixes (flagged in the Sunshine web UI).
CUDA/NVENC override carries over: verified the new build still probes
h264_nvenc / hevc_nvenc / av1_nvenc.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

flake.lock

@@ -674,11 +674,11 @@
     },
     "nixpkgs_5": {
       "locked": {
-        "lastModified": 1781074563,
-        "narHash": "sha256-md8WlXOlfnIeHeOScMTTHFyf2d6iaTwPl2apR5EQ3P4=",
+        "lastModified": 1782723713,
+        "narHash": "sha256-oPXCU/SSUokcGaJREHibG1CBX3+s/W7orDWQOZDsEeQ=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "9ae611a455b90cf061d8f332b977e387bda8e1ca",
+        "rev": "b5aa0fbd538984f6e3d201be0005b4463d8b09f8",
         "type": "github"
       },
       "original": {