[offline] Allow devbox to work offline if DNS fails

[mikeland73]

Summary

Fixes #1977

(with another hack).

Unfortunately the way we structure our flake makes it really difficult to remove our network requests (when state is stale). Ideally, the flake can be built off of local only nix store references, but that's a much bigger project.

In the meantime, this is a quick hack that fixes issue above.

How was it tested?

Added synthetic error that returns connection refused in the message. Ensures I was still able to run/shell with a stale state.

[savil]

what does the user experience if there was no cached environment?

i.e. can you rm -rf .devbox, turn off internet and see what you experience?

[mikeland73]

It fails with an ugly error. I just added a check so we only keep going if the print dev env cache is there.

There are still a few other edge case failure modes, e.g. if some package has been gc'd from nix store, but I think that's fine.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ *exec.ExitError: nix print-dev-env --json "path:": <redacted exec.ExitError> go.jetpack.io/devbox/internal/nix in (*Nix).Pri... View Issue
• ‼️ Generic Error: error running script in Devbox: GET https://search.devbox.sh/v2/resolve?name=go... go.jetpack.io/devbox/internal/searcher in execG... View Issue
• ‼️ **syscall.Errno: <redacted errors.withStack>: <redacted fs.PathError>: go.jetpack.io/devbox/internal/cuecfg in ParseFi... View Issue
• ‼️ **syscall.Errno: <redacted *errors.withStack>: <redacted errors.withStack>: <redacted errors.withStack>: <redact... go.jetpack.io/devbox/internal/shellgen in creat... View Issue

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