ensure gitea-runner owns its ubuntu state dir

The directory was created by the old DynamicUser (uid 63182). Add a
tmpfiles rule to guarantee correct ownership after any rebuild.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nixos/linux.nix

@@ -172,6 +172,7 @@ in {
     "d /root/.config 0755 ${username} users"
     "d /var/riverside/files 0755 root root"
     "d /var/riverside/postgres 0755 root root"
+    "d /var/lib/gitea-runner/ubuntu 0755 gitea-runner gitea-runner"
   ];
 
   networking.hostName = "${hostname}";