fix: nix hash update parsing... again (#18989)

.github/workflows/nix-hashes.yml

@@ -56,7 +56,7 @@ jobs:
           nix build ".#packages.${SYSTEM}.node_modules_updater" --no-link 2>&1 | tee "$BUILD_LOG" || true
 
           # Extract hash from build log with portability
-          HASH="$(grep -oP 'got:\s*\Ksha256-[A-Za-z0-9+/=]+' "$BUILD_LOG" | tail -n1 || true)"
+          HASH="$(nix run --inputs-from . nixpkgs#gnugrep -- -oP 'got:\s*\Ksha256-[A-Za-z0-9+/=]+' "$BUILD_LOG" | tail -n1 || true)"
 
           if [ -z "$HASH" ]; then
             echo "::error::Failed to compute hash for ${SYSTEM}"