[bug] plugin env PATH fix

[mohsenari]

Summary

When a plugin modifies PATH and also config in devbox.json modifies PATH via env:{}, the env overwrites the plugin and so PATH modification from the plugin is lost. This is because we do maps.Copy() to merge the env from plugin to the one from config.
The copy is fine for all other env variables but for PATH we need to handle merging the two rather than overwriting.
Fixes #2138

How was it tested?

bug recreate:

• devbox init
• devbox add ruby (ruby plugin modifies PATH)
• devbox shell (see PATH is prepended with ruby plugin)
• modify devbox.json and add "env": {"PATH": "/my/config/bin:$PATH"}
• re-enter devbox shell and see PATH prepend from ruby plugin is lost, only "/my/config/bin" is prepended.
  fix recreate:
• follow same steps, but in last step both PATH prepends from devbox.json and from plugin are present.

[mikeland73]

Can you use OSExpandEnvMap instead?

Basically take the environment provided by the plugins and use it to expand the environment of the config.

[mohsenari]

@mikeland73 after your suggestion, I tried OSExpandEnvMap and it it doesn't work.

The issue with OSExpandEnvMap is it merges the two PATHs correctly but it applies the (non-nix) system PATH to the :$PATH at the end. So with OSExpandEnvMap PATH becomes: /my/config:/my/plugin:/opt/homebrew/sbin:/usr/local/bin:...

With mergePATHsFromTwoEnvs PATH becomes: /my/config:/my/plugin:/Users/mohsenansari/code/go.jetpack.io/devbox/examples/stacks/rails/.devbox/nix/profile/default/bin:...:/opt/homebrew/sbin:/usr/local/bin:...

So OSExpandEnvMap can't work here because I don't want to expand $PATH env yet, I just want to merge them correctly without losing :$PATH suffix or $PATH: prefix to it.

[mohsenari]

Essentially, OSExpandEnvMap expands PATH with non-devboxified PATH value, but I don't want that.

[mikeland73]

Could you use OSExpandEnvMap instead of mergePATHsFromTwoEnvs?