Evaluate deploy output lazily

Currently, we evaluate the `#deploy` output strictly. This means
- Longer eval times
- Extraneous evaluation errors with `--skip-checks`
- `-- --impure` even when the path we're currently deploying is pure
- etc.

With this change, evaluation happens lazily -- we only evaluate the nodes
and profiles we really need. It is only implemented for flaky Nix, and
it is on by default. To get the old behavior, one can specify
`--strict-eval`.

I have tested that this indeed dramatically increases evaluation speed
in all of our repos, and removes the need to deploy Agora with
`--impure`. Hooray!

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