Is there a way to use only business days when calculating a field in GI?

Depending on how accurate you want your BuildDate, you can proceed with any of the below options:

If you just want Sat and Sun to be excluded:

Assuming your week is from Sun to Sat, then you can set up a conditional formula using DayOfWeek to get the BuildDate something like (you will need to play with the formula just I am giving the idea):

=DayOfWeek([AMProdItem.StartDate]) this formula gives you the day of the week a number between 1-7

Now if you are starting Monday then logically you expect to finish on Friday and if you start any other day you will have an additional Sat and Sun. So you can set up your BuildDate formula something like this:

1: Sun (Depends on your Calendar configuration)

2: Mon

…

7: Sat

=IIf(DayOfWeek([AMProdItem.StartDate])=1, DateAdd([AMProdItem.StartDate], 'Day', 6), IIf(DayOfWeek([AMProdItem.StartDate])=2, DateAdd([AMProdItem.StartDate], 'Day', 5), DateAdd([AMProdItem.StartDate], 'Day', 7)))

If you need to exclude Stat holidays other than Sat and Sun as well then you will customization as follows: 

Add a custom field to AMProdItem let's say UsrBuildDate

You will need to set up your Work/Manufacturing Calendar and identify the stat holidays.

In FieldSelecting Event Handler of your custom field, you will apply the Additional days logics and you will get the accurate date and return it.