Add nix- prefixed runner label, keeping old label for compat

Author: josh

pkgs/github-runner-config.nix

@@ -11,7 +11,10 @@
   github-runner-use-gh-token ? true,
   github-runner-group ? null,
   github-runner-name ? null,
-  github-runner-labels ? [ stdenv.hostPlatform.system ],
+  github-runner-labels ? [
+    stdenv.hostPlatform.system
+    "nix-${stdenv.hostPlatform.system}"
+  ],
   github-runner-work ? null,
   github-runner-ephemeral ? false,
 }: