[feature flag] respect env-var of feature flag (even if turned off) and default enable Nixpkg Version FF

[savil]

Summary

1. The Feature Flag library should respect the value of the env-var, whether ON or OFF.
2. Turn on the default value of the nixpkgs version FF.

How was it tested?

For change #1: respecting the env-var:

> DEVBOX_FEATURE_NIXPKG_VERSION=0 DEVBOX_DEBUG=1 devbox shell
> git status
# saw no changes to devbox.json as expected

For change #2: enabling the nixpkgs version

> devbox shell
> git status
# saw changes to devbox.json that adds the default commit hash

[savil]

Current dependencies on/for this PR:

• main
  • PR [feature flag] respect env-var of feature flag (even if turned off) and default enable Nixpkg Version FF #266 👈

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[LucilleH]

🚢 🚢

[savil]

from @Lagoja :

verified the feature, managed to downgrade my version of elixir in a project using: https://lazamar.co.uk/nix-versions/?package=elixir&version=1.8.2&fullName=elixir-1.[…]5d5da4abb74f0dd76967044735c70e94c5af1&channel=nixpkgs-unstable