Problem: Currently, the only way to test deploy-rs deployments is to
actually do a deployment to an existing NixOS instance (either in VM,
or a real machine) manually. This is a bit inconvenient and one can
forget to test changes when developing/reviewing deploy-rs changes.
Solution: Add NixOS VM tests.
Problem: node16 is now deprecated and github-runner provided by nixpkgs
no longer supports this runtime. However, "actions/checkout@v3" uses
this runtime.
Solution: Update CI pipeline to use "actions/checkout@v4".
Problem: We want to be able to run CI checks on PRs from external forks.
However, this is only possible with 'on: pull_request', while currently
CI is triggered 'on: push'
Solution: Change CI triggering condition to 'on: pull_request'.
