fix: increase parallelism for build process

We have reduced the pressure on CI runner, now the build process is timing out after 3h.
By increasing the parallelism from 2 to 4, we can better utilize the available resources and speed up the build process without overwhelming the system like before.

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 = 2
+          max-jobs = 4