.github/ci: Set max-jobs to 3

I believe this is a reduction, and the goal is to achieve a
reduction in peak memory usage, so that CI will pass again, and
possibly even be faster due to lower memory/cache pressure.

.github/workflows/ci.yml

@@ -23,7 +23,9 @@ jobs:
     - uses: cachix/install-nix-action@v30
       with:
         # The sandbox would otherwise be disabled by default on Darwin
-        extra_nix_config: "sandbox = true"
+        extra_nix_config: |
+          sandbox = true
+          max-jobs = 3
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
     - uses: cachix/cachix-action@v15
       if: needs.check_secrets.outputs.cachix == 'true'