fix(homeserver): ランナーの node20 を node24 へ向ける

nix/homelab/ci-runner.nix

@@ -62,11 +62,20 @@
       which
     ];
 
-    # This runner ships only externals/node24, while pinned actions still declare
-    # `using: node20` — the JS step then dies with ENOENT on a node that was never
-    # installed. GitHub's hosted runners remap it; this makes ours do the same instead
-    # of pinning every action to a version GitHub is retiring anyway.
-    extraEnvironment.ACTIONS_RUNNER_FORCE_ACTIONS_NODE_VERSION = "node24";
+    # Node 20 reached EOL, so nixpkgs dropped it and this package ships only
+    # externals/node24. Several actions we pin still declare `using: node20`, and the
+    # runner looks the runtime up by that literal name: the step dies before it starts,
+    # with ENOENT on externals/node20/bin/node. GitHub's own runners no longer honour
+    # ACTIONS_RUNNER_FORCE_ACTIONS_NODE_VERSION either (tried in #586, no effect) —
+    # they just run those actions on 24. So point node20 at node24 and do the same.
+    #
+    # Chasing every action to a node24 release instead would be a moving target: the
+    # installer and cachix actions are pinned by SHA on purpose.
+    package = pkgs.github-runner.overrideAttrs (prev: {
+      postInstall = prev.postInstall + ''
+        ln -s ${pkgs.nodejs_24} $out/lib/externals/node20
+      '';
+    });
 
     serviceOverrides = {
       # Half the machine, at most, for everything outside the daemon.