[envsec] Add envsec to path if used in config

[mikeland73]

Summary

This PR adds envsec to devbox env $PATH if it is used in config. This allows user to use envsec without explicitly installing it and without adding any envsec specific commands to devbox. e.g.

devbox run envsec ls

and devbox shell followed by envsec ls

will both work.

Other improvements:

• Don't modify current process path when adding envsec (it's best to avoid os.Setenv if we can so that environment is more consistent trough run and now longer depends on when envsec.EnsureInstalled is called.)
• Remove feature flag (this feature is gated anyway by requiring explicit opt-in in the config)

How was it tested?

devbox run envsec ls