There is no extension install UI in a totally empty wiki right now. You can only import pages, but it’s not enough to install a complex dependency tree like the standard flavor.
You can delete the status which store the fact that you don’t want any flavor. It’s located in <permdir>/jobs/status/distribution/status.xml.zip (/var/lib/xwiki/data/jobs/status/distribution/status.xml.zip with the Debian package)