flox --help is missing build when feature flag is enabled

[ghudgins]

Acceptance Criteria

• Enable build feature flag
• flox help

Expected: build command in help Actual: no build command in help

[ysndr]

help messages are statically generated and thus cant be changed by a runtime feature flag (at least not with significant duplication). That's in fact similar to our manpages. Personally i think we could have a section in the ehlp output that does show experimental commands but clearly labels them as subject to change. Similarly, we could then also include the manpages.

[zmitchell]

flox build is now stable