[nix.System] call EnsureNixInstalled from devbox.Open, and cleanup function API

[savil]

Summary

nix.System is widely used. So, in #1357 we wanted to change its API to just
return string instead of string, error.

To do so, we cache the result in a nix package variable. And populate it
during EnsureNixInstalled.

However, EnsureNixInstalled was only called from Cobra command functions. But
devbox as a library also now needs to call it. IMO, it should always have called
it since we do rely on nix being installed. This PR adds this.

It also fixes up the API to:
System: string and ComputeSystem: (string, error).

NOTE: inside System, I still do a best-effort to avoid panic by calling
ComputeSystem. This can happen in edge-cases like some tests generate_test/TestWriteFromTemplate
that exercise internal functions reliant on nix.System without calling devbox.Open.

How was it tested?

tests should pass

[savil]

Current dependencies on/for this PR:

• main
  • PR [nix.System] call EnsureNixInstalled from devbox.Open, and cleanup function API #1374 👈

This comment was auto-generated by Graphite.

[mikeland73]

ComputeSystem should only return error (we always discard the string)

[savil]

I had to bump the test-timeout to land this. We needed this PR for correctness. But I will pursue a fix to improve the test times.

[sentry-io]

Suspect Issues

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

• ‼️ **exec.ExitError: <redacted errors.withStack>: <redacted exec.ExitError> go.jetpack.io/devbox/internal/nix in System View Issue
• ‼️ **exec.ExitError: <redacted errors.withStack>: <redacted exec.ExitError> go.jetpack.io/devbox/internal/nix in System View Issue

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