nixos/gitea-actions-runner: set the $HOME environment variable

This fixes an issue where running the official `actions/checkout` action
from GitHub on a runner with label `:host`, would fail with incredibly
unhelpful error messages.

Turns out, this is due to `$HOME` not being set.

nixos/modules/services/continuous-integration/gitea-actions-runner.nix

@@ -203,6 +203,8 @@ in
             TOKEN = "${instance.token}";
           } // optionalAttrs (wantsPodman) {
             DOCKER_HOST = "unix:///run/podman/podman.sock";
+          } // {
+            HOME = "/var/lib/gitea-runner/${name}";
           };
           path = with pkgs; [
             coreutils