[flakes] reset nix profile directory when Flakes feature is toggled

[savil]

Summary

If a nix-profile-directory was created for an old nix-env profile, then when
trying to use the new flake-y nix profile with the same directory, an error is
reported.

This PR enables us to toggle DEVBOX_FEATURE_FLAKES= on and off for the same
devbox-projects by inspecting the contents of the profile:

1. a manifest.nix means that its an old style profile
2. a manifest.json means that its a new style profile (i.e. to be used with flakes feature)

The code will "reset" i.e. delete the directory and re-create it.

How was it tested?

inserted a print statement prior to os.Remove(dir) in the reset function

> DEVBOX_FEATURE_FLAKES=0 devbox shell

> DEVBOX_FEATURE_FLAKES=1 devbox shell
reset = true

> DEVBOX_FEATURE_FLAKES=0 devbox shell
reset = true

Observe:

1. no error, we can toggle seamlessly
2. only one reset = true printed despite multiple calls to devbox.profileDir()

[savil]

Current dependencies on/for this PR:

• main
  • PR [flakes] reset nix profile directory when Flakes feature is toggled #612 👈

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