We took a slightly different approach for tumbleweed and dropped
support for the pip-and-virtualenv element for it (and Fedora 31+)
with I7a6a342461d6001c25e55638ba9b7438c28f2519. So there's no need
for the -plain image now; switch the main image to using the base pip.
This is currently failing to build due to this.
Change-Id: I3333b475ff9d2f2fb96ad5588c6250cba2593e06
368a8983e5