fix: reduce pressure on CI runners

Updating the dependency helping us build rust extensions has put a lot
of pressure on our CI runners, causing timeouts and failed builds.

We are currently working on a more permanent solution in #1745 but in
the meantime, we cap the number of parallel nix builds to 2, which
should reduce the load on our CI runners.

Author: jfroche

.github/actions/nix-install-ephemeral/action.yml

@@ -44,4 +44,4 @@ runs:
           substituters = https://cache.nixos.org https://nix-postgres-artifacts.s3.amazonaws.com
           trusted-public-keys = nix-postgres-artifacts:dGZlQOvKcNEjvT7QEAJbcV6b6uk7VF/hWMjhYleiaLI= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
           ${{ inputs.push-to-cache == 'true' && 'post-build-hook = /etc/nix/upload-to-cache.sh' || '' }}
-          max-jobs = 8
+          max-jobs = 2