Modify the product variable Build_from_text_stub value

productVariables.Build_from_test_stub is currently being set by the
config.buildFromTextStub value. However, this leads to divergence in the
behaviors between the exported BuildFromTextStub() value and the config
value, as the former depends on other factor (whether is it a coverage
build). This change fixes the divergence by making the product variable
value to be set by the former.

Test: m nothing
Bug: 301522358
Change-Id: Ic4de5a179dd1094eb8788663e4d6afa4bea724ea