Use Nix for builds on cdk-environment EC2

[patchwork01]

Background

For development, we currently use a Docker container on the EC2 instance deployed by the cdk-environment module. That EC2 instance has the Sleeper CLI installed automatically, and we use the sleeper builder command to get a Docker container for builds and deployment.

We may want to deprecate that Docker image, and replace the Sleeper CLI with a version that runs directly in Java. One reason for this is a bug in GitHub Container Registry:

• Sleeper CLI Docker images on main branch can't be pulled #2494
• Plan approach to GHCR image problem / CLI / Docker images #2684

Description

We'd like to convert the build EC2 deployment in the cdk-environment module to use Nix to set up a build environment.

Analysis

We can install Nix as part of the cloud-init script for the build EC2.

We can remove the automatic installation of the Sleeper CLI. In practice we only use the current CLI to deploy and connect to the environment / build EC2, and to get a build environment on the build EC2. We won't need it on the build EC2 if we use Nix. We could leave that for a separate issue.

We can convert the nightly system tests cron job to run in nix-shell instead of a sleeper builder Docker container. We could leave that for a separate issue.