[Devbox] introduce devopt.EnvOptions

[savil]

Summary

From the EnvOptions docblock:

// EnvOptions configure the Devbox Environment in the `computeEnv` function.
// - These options are commonly set by flags in some Devbox commands
// like `shellenv`, `shell` and `run`.
// - The struct is designed for the "common case" to be zero-initialized as `EnvOptions{}`.

This gets rid of the pseudo-global state in the Devbox struct where we were setting pure, preservePathStack and omitNixEnv values.

How was it tested?

TODO:

• [x] CICD should pass
• [x] Ensure devbox global and devbox omitNixEnv works as expected
• [x] Ensure pure works as expected for devbox shell --pure.

[sentry-io[bot]]

Suspect Issues

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

• ‼️ Generic Error: <redacted errors.withStack>: <redacted usererr.combined> go.jetpack.io/devbox/internal/boxcli/usererr in... View Issue
• ‼️ syscall.Errno: <redacted errors.withStack>: <redacted fs.PathError>: go.jetpack.io/devbox/internal/cuecfg in WriteFile View Issue
• ‼️ Generic Error: <redacted errors.withStack>: <redacted usererr.combined> go.jetpack.io/devbox/internal/boxcli/usererr in... View Issue
• ‼️ Generic Error: <redacted errors.withStack>: <redacted usererr.combined> go.jetpack.io/devbox/internal/boxcli/usererr in... View Issue
• ‼️ *exec.ExitError: <redacted usererr.combined>: nix path-info exit code: 1, output: <redacted []uint8>, err: <redac...** go.jetpack.io/devbox/internal/nix in StorePaths... View Issue

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