actions-runner: fix cache dir for DynamicUser

Move cache directory under the managed state directory since the
service runs with DynamicUser and cannot create /var/cache paths.

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

services/actions-runner/default.nix

@@ -23,7 +23,7 @@
         runner.capacity = 2;
         cache = {
           enabled = true;
-          dir = "/var/cache/forgejo-runner";
+          dir = "/var/lib/gitea-runner/actions1/cache";
         };
         container.privileged = false;
       };