There seems to be no way to change the button position of any race’s build ability. I’ve tried commandfunc.txt and setting it through triggers, and neither way works. Commandfunc will switch other button positions without any problem, but will not move the build abilities.
To work around this for personal use, I made a spellbook that uses the build order string so that it contains the entire build menu. That way, I can have the same thing and am able to move the button position of the menu. I found out that it is possible to hide/disable the real build ability via trigger, so I did that on top of my spellbook ability so that there’s only one build menu. However, hiding the real build ability causes all buildings to disappear from the fake build ability (the spellbook). I feel like none of this is really intended behavior, so I thought I should share.