Improve implementation of fix for unbuilt units not being destroyed in factories when mobile build orders are cancelled

[lL1l1]

Description of the proposed changes

• Only run code for the edge case when the build does not fail but the unit is incomplete.
• Use proper callbacks instead of relying on the normal finish build behavior to stop build fx and destroying the unit manually

Testing done on the proposed changes

Same as in #6700:

An unfinished unit no longer remains in the factory after the mobile build order is cancelled.
Copy-paste command for testing:

CreateUnitAtMouse('urb0301', 0,    0.00,    0.00, -0.00000)
ForkThread(function() 
WaitTicks(10) 
ConExecute('UI_SelectByCategory +nearest FACTORY CYBRAN TECH3')
import('/lua/ui/game/commandmode.lua').StartCommandMode('build', {name = 'url0303'}) 
end)

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in a changelog snippet according to the guidelines.
• Request 2-3 reviewers from the list of reviewers and their areas of knowledge.