ci: Remove max-jobs = 1

This change was necessary when we were using `nix flake check` for CI
(see 6db6b269ed). Now this is not really
necessary, because we are running the checks in a much saner way, that
doesn't use up too much memory for evaluation.

.github/workflows/ci.yml

@@ -56,9 +56,7 @@ jobs:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         dogfood: true
         # The sandbox would otherwise be disabled by default on Darwin
-        extra_nix_config: |
-          sandbox = true
-          max-jobs = 1
+        extra_nix_config: "sandbox = true"
     - uses: DeterminateSystems/magic-nix-cache-action@main
     # Since ubuntu 22.30, unprivileged usernamespaces are no longer allowed to map to the root user:
     # https://ubuntu.com/blog/ubuntu-23-10-restricted-unprivileged-user-namespaces