fix(ci): serialize linux aggregator builds

The Linux CI run on e7c2d39 fails with `No space left on device` while
openclaw-gateway, openclaw-gateway-tests, and openclaw-config-options are
building together on the GitHub runner.

Limit the Linux aggregator to one Nix job so those heavy derivations do
not peak disk usage at the same time.

Tests:
- verified remote failure in CI run 24211735739: error writing to file: No space left on device

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
         uses: DeterminateSystems/nix-installer-action@v13
 
       - name: Run Linux CI aggregator
-        run: nix build .#checks.x86_64-linux.ci --accept-flake-config
+        run: nix build .#checks.x86_64-linux.ci --accept-flake-config --max-jobs 1
 
       - name: Dump failing gateway test log
         if: failure()