software/slapos-testing: add back required versions pins

217f7f05 (slapos-testing: don't pin tested eggs, 2021-01-12) was wrong,
some versions needs to be pinned for initial installation of the software.
The problem was not visible when the software was already installed and
updated in place, but when installing software from scratch, it was failing
very early with an error like:

    While:
      Bootstrapping.
      Getting distribution for 'slapos.libnetworkcache'.
    Error: Picked: slapos.libnetworkcache = 0.20