ci: speed up on no-op

.github/workflows/nix.yml

@@ -12,4 +12,4 @@ jobs:
         extraPullNames: nixpkgs-update
         name: nix-community
         signingKey: ${{ secrets.CACHIX_SIGNING_KEY }}
-    - run: nix-build
+    - run: ./ci.sh

ci.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# Run this command to reproduce CI
+set -euo pipefail
+
+cd "$(dirname "$0")"
+out=$(nix-build nix -A nix-build-uncached)
+
+"$out/bin/nix-build-uncached"