There is no first class support for this anymore in 2.0
You can read about this and other changes here: https://circleci.com/docs/2.0/migrating-from-1-2/#search-and-replace-deprecated-20-keys
The following command should work as a workaround.
run: echo 'export BS_BUILD="Build No. $CIRCLE_BUILD_NUM for CircleCI"' >> $BASH_ENV